Bears and Beavers

OG BEARS
Bears are notable due to their additional 'hug' attack if one of their two paws scores a hit with an 18 or better (unmodified I assume) they hug for additional damage, and the larger varieties of brown and cave bears fight for 1d4 melee rounds after reaching 0hp, but die instantly if reduced to -9hp or lower. Polar bears, introduced in the MMII, fight on for 1d4+1 rounds and die instantly only at -12hp, but are otherwise unremarkable in simply being bigger and hard hitting.

They have 'excellent hearing and smell, but rather poor eyesight' which encourages ranged attacking (though to be fair, anything without a ranged attack encourages ranged attacking) and are otherwise pretty much just your classic animal statline- lots of HP but not much AC, and enough damage to make 'surround and stab furiously' a plan that will probably make at least one person die. They also travel in packs of 1d6, which means a bear encounter could potentially offer maulings for the whole party.

OG GIANT BEAVERS
 Dealing 4d4 damage, with 4hd, numbers of 10-40, and the amphibious terrain advantages from dwelling in the middle of an artificial lake, frontal assaults against beaver dams are unlikely to go well, and their sodden lodges make fire unlikely to be effective. Interestingly, they are intelligent creatures, rather than 'just' animals. They prize birch, aspen, and willow bark as well as coins and other valuables, and can be hired to create dams in watery areas. While their treasure type is unreliable, their hides are worth 500 to 2000 gold pieces each, and their young can be sold on the market for an average 1000gp each each meaning an average beaver lodge is worth 40,000gp if your party has no qualms about slaughtering 'intelligent and docile' creatures and selling their children as... pets? Slaves? Why is there an assumed market for giant beaver children, of all things?

That said, a lodge of them could be interesting to place in a wilderness region the players are attempting to build a castle in. The beavers could be a handy ally, allowing the players to shape the region by diverting rivers, flooding valleys, and similar landscaping endeavours, either by perhaps hunting the beavers predators as a favor or simply paying them as contractors. Or, I suppose evil groups could kill them all for money, set a wicked bunch of amphibious demi-humans to use the lodge as a castle, etc etc.

A Yuban sloth bear, a grizzly of more temperate regions, and probably suspicious dealings between a Mercian wizard and a giant beaver

SUNSET REALM BEARS
The Noonlands are too hot for bears, but they are common enough elsewhere, be it the jungles of Yuba and the Beast Islands, the more temperate Mercia and Prince's Spit, and the Moonlands in general, even the deep moonlands that are permanently cold (though white 'polar bears' are found only in the Wrecker's Coast and the Auroral Reaches- Moon Bears, using the Cave Bear statistics, are grey to blend in with whatever moonlight is currently shining). Bears have some of the most powerful souls of all animals, clinging to life even as the body is broken, and so necromancers who value strength and power over obedience may seek out bear-infested wildernesses to harvest minions. The spirits of mother bears taken from their cubs are easy to manipulate via simulating their cubs being in danger, be that via actually dangling the cubs in a cage, illusions of such, or the bodies of the undead cubs being controlled by more easily directed souls.

SUNSET REALM GIANT BEAVERS
With the  gross overabundance of strange furry monsters that roam the realms, beaver pelts are not as treasured as they were in real life, and mostly live lives unmolested by humans save for the occasional mountain trappers who truly appreciate water-resistant furs. As animals are all sort-of as smart as people just with different priorities, the intelligence of giant beavers is not usually remarkable, but their ability to do mega-projects with water has led to some interaction with humans on more than a interpersonal scale. The mountains separating King's Point from Queen's Coast are home to some lodges of giant beavers, as are the Mountains of Mercia. In the former case, the beavers are often hired to divert rivers to spite the opposing side and occasionally VIPs seek refuge among beaver lodges instead of mountain monasteries. In the mercian case, humans have hired the beavers to create scenic lake castles (the castle being constructed in a dry but otherwise unhospitable valley beforehand), and dwarves have commissioned reservoirs high in the mountains in the event subterranean water supplies become unsafe for whatever reason. Most notably, the fortress  Stonefast Two had an entire mountains worth of snowmelt diverted to drown (or at least, contain) whatever darkspawned horrors had risen from the depths, and the giant beavers have been under the personal protection of the Underlords ever since and hunting them or interfering with them is forbidden by dwarvish law, though Mercian rebel warlords have risked dwarvish retaliation to destroy mountain dams and unleash water on the unsuspecting lowlands more than once.