A truly incredible array of dinosaurs is detailed in the AD&D Monster Manual- over two dozen of the beasts are detailed, but unfortunately, none of them are really all that interesting. They have massive HD and damage, but usually only moderate AC, making them fairly vulnerable to swarm tactics from, say, 20-200 brigands with slings, but quite likely to be able to eat distressing proportions of the average adventuring party. They are described as being extremely stupid, so perhaps they were meant to be a sort of 'fight smarter not harder' lesson for players to learn. This lesson is often taught with mindless undead, but big angry dinosaurs could provide similar lessons in trickery, evasion, and control of terrain before introducing similarly big nasty monsters that would be far more likely to TPK ill-prepared parties, such as dragons and giants.
That said, there's nothing particularly interesting about any given entry aside from the occasional truly ludicrous set of numbers (I used to amuse myself by flipping through the monster manual and noting which monsters could be squashed in a single step from a brachiosaur, Lolth being 'demon lord voted most likely to be crushed by a sauropod'). The dinosaurs could have easily have been covered with an arbitrary 'roll 2d10 for HD, pick a random armor class and give anything with plate armor class a weak spot, then add an attack pattern fierce enough to shred any given adventurer in one round' and you'd basically have more than enough to run all the dinos anyway.
Rather sensibly, 3.5 cut down on the number of dinosaurs, though I question the choice of keeping 3 different sizes of theropods and leaving out classics like sauropods and pterodactyls. Unfortunately, the space saved by removing dinosaurs was immediately spent on 'Dire Animals' which are similar blobs of big combat maths tethered to animal stupidity, and might as well just be furry dinosaurs.
Sunset Realm Dinosaurs
The reptiles that sided with Yg became snakes and traded their legs for wisdom, but those that kept their limbs and their neutrality became huge, powerful, and not all too intelligent. As such, the cunning snakes soon ruled over the dull beasts and domesticated them, much as humans live with cows and dogs and so on. However, the dinosaurs became mostly extinct when the reptiles that aligned with Yg-A came down from the sky- the fire-dragons. The surface was scoured by flames, and huge beasts like dinosaurs were either eaten by the ravenous dragons, or starved as the massive amounts of food needed to keep them alive vanished in flames.
Nowadays, dinosaurs live only on forgotten islands that were sheltered from the rain of fire, in underground mushroom caverns, and deep in the moonlands that are favored by Spring, the unnatural plant growth feeding the bellies of the huge creatures, who in turn feed the bellies of their carnivorous predators.
Due to their immense size but relative simplicity, they are treasured by Beast Battlers who value strength, and by necromancers who want massive corpses, and by nobles who want a huge and impressive beast to show off, but not anything that's too likely to escape and cause terror.
OG Displacer Beasts
A classic monster, sworn enemy of blink dogs, and basically just a panther with tentacles and a mirror-image style ability to help it avoid attacks. Honestly I always thought it's trick deserved a bit more than a -2 to hit it for appearing 3' from it's actual position, but whatever. They also have very good saves, saving as a level 12 fighter +2 against magic, presumably due to magic 'missing' them.
They don't have very interesting motives past the blink dog rivalry, as they "hate all life," which seems a bit extreme for a semi-intelligent animal with a neutral alignment.
It's somewhat interesting that they only attack with their tentacles rather than their puma-ish claws and jaws- after some thought I assume this is to perpetuate the advantage of the displacement visual effect. Flailing tentacles are harder to isolate the source of compared to 'ooh argh there's a puma on top of me eating my face.' I think they should have a claw/claw/bite routine available, it's just that they lose the displacement advantages if they lose it.
3.5 Displacer Beasts
With a flat 50% miss chance if you don't have some non-sight based means of locating where they are, I actually like how the displacement is handled more here.
|my brother had a nightmare about walking puma once so that's where I got the idea|
Sunset Realm Puma-That-Walk
It is well known(by nerds, anyway) that cats can traverse the mirror realm and the netherworld. What is less known is that their reflections are deeply jealous of this ability, and seek to steal one of their sources nine lives to give them free rein to walk into the waking world. This is why cats do not always recognize their own reflections and can be spooked by mirrors. Much like the violation of the natural order that leads to tigers and rakshasha, this unnatural theft creates a monster, a Puma-That-Walks.
Though statistically they are identical to other big cats, they walk on their hind legs and have darkened fur and blank white eyes, so their monstrous nature is unmistakable. They are as cunning and wicked as tigers, though as evil reflections of cats they are less earthy and more nightmarish. In the wild, they emerge dry from still pools, and can leap back into them to escape hunts gone wrong. But it is their ability to appear from glass and other mirrored objects in the heart of civilization that sets them apart from mere wilderness beasts. They hunt their source cat primarily (gaining 1hd per stolen life from the cat), and incidentally prey on vulnerable targets such as noble children and elderly retainers while guards and knights are away on hunts and the like(or in rural areas they could eat shepherds and so on, haunting ponds). Ideally they lure their prey into the mirror realms before delivering the final blow. Cats and Dogs are aware of the existence of these horrid beasts and can be counted on for help when humans are shackled by skepticism, and Very Good Dogs are sometimes even dispatched to rid households of Puma-That-Walk.
Rather than a displacement effect, they have the ability to enter and exit through mirrors, and these mirrors remain portals until broken (ripples count for ponds) or covered up and placed into darkness, allowing mere mortals to traverse the realms as well. They can be combated while in the mirror realm by looking into a mirror to locate them, though this is quite difficult and probably has 50% chance to fail due to reversal of left-right and so on. However, while you are reflected your reflection is vulnerable as well, and can be harmed in kind, though exiting the reflection breaks the link. It is worth noting that when a mirror becomes a portal it is not showing a reflection, but is simply like looking through any 'normal' aperture.
So the difficulty is thus- in the provided image, for example, if the mirror is a portal, the cat is indeed 'in front' of our knife-lady. If it is not, a portal, the cat is indeed 'behind' her and can menace her by attacking her reflection, and vice versa. You can, of course, simply break/cover all the reflective surfaces you find and avoid this tricksome dilemma, but!
1. Mirrors are valuable as loot.
2. The cat can probably find some forgotten puddle faster than you can seal off all possible apertures to the mirror realm.
3. Running away from your problems sometimes solves them, but in the case of evil mirror cats with a penchant for stalking, ambush, and vengeance, putting off the problem till later when your guard is down might be more dangerous than trying to end things now.