Skiurid

Yes, D&D has an evil squirrel.

Part of the wildlife of the Shadow Plane, Skiurids usually live in colonies of up to two dozen individuals, along with their pups. Like other natives of their plane, Skiurids are capable of summoning magical darkness. The squirrels create cold areas of darkness, which drain the energy from anybody who happens to wander into them. That drained energy solidifies into a black nut, which the squirrels collect when the coast is clear. Their dens usually contain a small stockpile of nuts for pups and for rainy days. Skiurid energy nuts are coveted by necromancers, who use them to empower their spells.  (In text rules, 50% chance of raising the caster level of a necromancy spell by 2).

The thought of evil squirrels is pretty ridiculous, so much so that Skiurids are better suited for a comedy game. I can imagine evil squirrels in Adventure Time. Or if you just want to mess with your players.

I think it works a bit better if you think of it a bit like the Elysian Thrush. Both are very low CR creatures that would probably only attack someone if directly threatened, but are capable of some fairly impressive environmental effects. For all your PC knows, they've just walked into a patch of darkness caused by some more threatening foe. And only if they make a fairly high perception check will they notice a little black squirrel picking something off the ground.