Giant Gar, Goyle Gar, and Gas Spore

 AD&D Giant Gar
This is just a big 8HD, AC as plate, danger fish that swallows you whole on 20's and has a 5d4 bite.
There are also giant Pike later in the book, but honestly I feel like those entries (and any other oddly specific giant fish) entries could probably have been condensed to a single 'Giant Fish' section, or even been a footnote in the 'Shark' section mentioning 'the shark statblock can also serve for various predatory fish.

AD&D Gargoyles
Gargoyles in AD&D are pretty nasty sorts, being immune to nonmagical weaponry (though their AC is merely as Chain), boasting numerous but weak attacks via claw/claw/bite/horn gore attack routine, and of course flight. They are 'predators of a magical nature' that are 90% likely to attack anything they detect, and are said to enjoy torturing prey to death on account of them being evil monsters. It is not at all clear if they are evilly animated statues intentionally created or some manner of naturally occurring stony monster, but their presence in underground caverns as well as ruins points to the latter, while later modules depicted gargoyles that seem directly inspired by stonework.

They also have, bizarrely, an aquatic variant known as the kopoacinth, which swim with their wings and are found in shallow waters with sea-caves.

Though I prefer a friendlier take on these creatures, I think they are pretty good monsters to serve as minions for larger villains (as the book mentions.) They are immune to normal weaponry, making them good at terrorizing townsfolk, flying away with hostages, and providing a demon-like threat without the grab-bag of magical powers actual demons have.
BFRPG Gargoyles
Largely identical to their origin, this interpretation leans more on them being 'evil rocks' giving them better camouflage/surprise chance when they stand motionless and pretend to be statues, and their predations are confirmed to be cruel in nature, as they need neither food, air, water, or sleep.

DCC Gargoyles
While similar to the BFRPG variant, they have AC as Plate+Shield+3 but only 2HD, and normal weapons still inflict damage, but shatter upon striking the gargoyle 50% of the time. This makes them more prone to being 'seemingly invincible, then suddenly shattered' and a drain on weaponry that better represents them being made of stone but not magically resilient, and all in all I think is a better take than the 4HD gargoyles which are a bit of a slog to get rid of if you actually have the capacity to do so.

AD&D Gas Spore- A nasty sort of 'trick' monster which resembles a beholder, but is in fact a sort of lethal exploding fungus balloon. Killing one causes it to explode in a 6d6 blast in a 20' radius, which even with a save for half is pretty rude as a gotcha moment.

Interestingly, despite being an order of magnitude or three weaker than a beholder, I feel like they can only be used properly AFTER the party has encountered a beholder, for that gives the party a chance to realize something is off (primarily the fact that the main eye of a beholder should be cancelling magic as it gazes upon the party, while a gas spore's 'Eye' is just fungal texture.)

Once the functioning of gas spores has been realized, they could become interesting encounter complications akin to an exploding barrel in DOOM, or terrain hazards in rooms with high bridges, narrow walkways, tight halls, and so on.

One thing I had quite forgotten until rereading the entry is that letting them bob balloon-like up to someone, apparently harmless now you haven't detonated it, is in fact another 'gotcha' moment, where the gas spore will 'shoot tiny rhizomes' into someone, die on the spot (without exploding), and infect the target such that they will die in 24 hours and sprout into 2d4 more gas spores. While a fascinating fungal life-cycle, there's no reason to expect an exploding ball to suddenly function as a hypodermic needle injecting its fungal contents upon touch (or so I imagine the process) and so I'm not too fond of the double trickery of this creature, neither of which is something you would expect.


Sunset Realm Giant Fish
Fish can grow giant just like everything else, but I have no special attachment to Gar, and honestly have plenty of other sea-beasts I'd rather use.

It is a common belief that the uglier a gargoyle is, the more fearsome a guardian it will be

Sunset Realm Gargoyle
Gargoyles come in two varieties- church-defenders carved from the hallowed stone of a temple, their elemental souls given humanoid egos by their new shadows. This is a High Alvish technique adopted first by the 3rd solar age sorcerer-king Sarkomand the Omnipotent, then by the 4th age Church of the Stone Sun. In the absence of either, remaining gargoyles tend to be ancient and somewhat battered, lurking in old ruins and defending them for no good reason anymore, making it not at all impossible to negotiate with them if the appropriate ancient language can be utilized and at least some moral similarities to their fallen cult demonstrated (destroying undead is usually a safe bet, as watching over church graveyards to keep the dead under control was a common task, and recovering lost relics, stained glass windows, and so on that the gargoyles liked to gaze upon is another good bribe.)

The other, more sinister origin of a gargoyle is one who is carved from a (preferably extremely obese) humanoid petrified by the knowledge of Yg transmitted via Basilisk or Medusa, reshaped into an entirely new being by the snakey folk responisble. The original soul is usually cast from the body in this process (unless it hangs on out of consent or stubborn determination), the new soul of a gargoyle being an artificial shadow-soul tweaked to the specifications of the creator. Medusa like gargoyle servants, for they are immune to petrification, and they featured predominantly as soldiers in the Oroboron Civil War in which the Serpent Queen Tinnea (Herself a descendant of Vala) tried to take the city from King Samuel Goffnagoff, only to be ousted in open revolt from basically all the human nobles of the land and replaced by Queen Evalyn Goffnagoff, daughter of the king. This event marked the addition of the goddesses Yg and Lumar to the list of unsanctioned cults of the realm, which stands to this day.

That aside, gargoyles are decidedly an urban creature, (either inhabited or ruined) and the church-defending sort tend to know quite a lot about the goings on of priests, for they have nothing to do but gaze stoically from their perches and gossip as they wait for their time to act. As such, investigations of corruptions sometimes bear unexpected fruits when the ancient gargoyles of an abbey are consulted, and truly unforgivable goings on may even bring down the gargoyles of a building to depose of problems. This is not exclusively limited to religious buildings- some gargoyles were built for military defense and surveillance and do away with spies, thieves, and saboteurs, and the religious leanings of gargoyles is a matter of the secret techniques being passed down among the gods enslaved by the Stone Sun pantheon and therefore used in religious projects far more often than anything else.

Sunset Realm Gas Spore- Fungus is poorly understood by scholars, but it is generally agreed that it is the 'plant life' of darkness, in contrast to sun-loving trees and herbs. And while monstrous plants are not unheard of (the twin moons Spring and Autumn producing such vicious shrubbery without fail), it is no surprise that dark-thriving fungus grows into monstrous forms in the dark.

Gas spores are a form of fungus that grows on rotting flesh and other organic matter not defended by the light of life, and they tend to resemble what they grow from, leading to spores that look like human heads, skulls, bloated roadkill balloons, other mushrooms and so on. The similarity to beholders is more a matter of them being 'a spherical floating something' than true mimicry, but the dim lighting and twitchy reactions of dungeoneering typically works in their favor. They are attracted to air currents and slowly fly about by ejecting spores as propulsion, and they explode when violently struck, or when they touch something that registers as 'alive' or at least 'organic' to their mysterious fungal senses. The concussive burst is sometimes lethal due to throwing those caught in the blast to crack their skulls on rocks and so on, but mostly just knocks people prone with some blood coming from the ears and nose, perhaps attracting nearby monsters (1d6 damage, save vs dragon breath for half, per 1' diameter of the spore, the damage dropping off by 1d6 every 5 feet of distance from ground zero. Spores grow to a maximum diameter of 6' before exploding themselves). Anything spore-covered will infect any dead organic matter it touches until either they clean off the spores, or the spores perish in sunlight.

Infested organic matter (such as the rations of a spore-covered adventurer) will look and taste moldy at first, and over the course of the next 1d6 hours will collapse into rot and sprout into a new gas spore. Corpses will provide enough energy to produce 2d4 gas spores. Infested wood will sprout gas spores as well, but at a slower rate, until burnt. Rotten dungeon timber tends to serve as a sort of 'spawner' of gas spores, ensuring a dungeon will have wandering gas spores until all the infestation points have been destroyed.

The idea behind this variant of gas spore, from GMing perspective, is to have them serve as an explosive menace that also serves as ration and corpse depletion, and built in dungeon re-stocking. The players might turn the gas spore life cycle to their advantage, or fail to take appropriate measures and find themselves drowning in the things as everything in the dungeon falls.

Explodestools- Exploding land-mine toadstools were a favorite of mine, depicted in a dragon magazine article I forget the name/issue of. I think a variant gas spore that grows as a bloated mushroom rather than a floating balloon makes for a good potential change, with the major difference being that Explodestools can thrive aboveground, and of course are stationary hazards.