As a weasel fan, I feel the Mustelids in the game have been grossly mistreated. Not only do they all only get one sprite, in the game files it is labeled 'rodent'! Rodents are what weasels
eat, not what they are! Preposterous.
Jokes aside, I do feel like they need some work. First of all, by 'mustelids' I mean the small ones - weasel, ermine, polecat and pine marten. The glutton and badger have their own sprites and behaviors that are just fine.
I think, above all, they need to be more common. A character can play several years without seeing a single weasel or polecat. I understand that these animals are small and secretive, but that seems a bit much - they shouldn't be so much rarer than foxes. These animals' furs are among the most expensive, by pound of weight, so I understand their rarity may be intended to balance it out, but with the weight of a single ermine hide being what it is, it's only worth as much as a common hare's. They could stand to be easier to find, especially with the trade in their valuable furs having been so historically important.
Having your loop-snared birds nibbled by a stoat should be a real danger, I feel.
Pine martens should climb trees, similar to squirrels. Perhaps with a less stupid AI -- make them harder to catch, perhaps they can climb down trees if stuck.
Despite my extensive setting of loop snares with meat bait, I have only caught carnivorous birds in them. It's hard to find information on whether mustelids can be caught in these in real life - at least one source says martens can, but this may be pertaining to modern, wire snares, that they cannot chew through.
My next suggestion is for a new species, an absent European mainstay - the otter. They could use a similar AI as beavers, except for occuring alone and being baited by fresh fish and meat. The european mink is another suggestion. In today's Finland they seem to be very rare if not extinct, but historically they were widespread. They're another waterside animal, but not much of a diver unlike the otter.
Lastly, if there is such a need or desire, I would like to offer to recolor the sprites to match individual species myself. It would be a great honor to contribute in this small way to a game I have been playing for many years.
I attach a sample polecat