Moss Adder! They’d spawn in lush caves, and be a bit of a trap. If you’re not careful you could get poisoned, unless you spot it first and offer a raw rabbit. Once you feed it, it’ll gladly hop into a decorative pot and poison mobs that come across it.
I’m in the process of making a collection of models for a potential mod: Rivers & Lakes (title W.I.P)
The Beaver would make the world a little more dynamic by acting as a forest trimmer. They’d chop down trees, and when the decayed leaves drop a sapling it would automatically plant. The beavers would spawn in groups of 2-3 around a dam. The dam, acting as storage for all the harvested wood, as well as the home of another mob that works together with the beavers.