There are little Thaumic slimes that are smaller than normal slimes and grow bigger if they fall into a crucible.(thaumic slimes are only in thaumcraft 2) They do damage despite their size. There are also whisps that are neutral energy beings that drop vis crystals.

Also there are brainy (or smart) zombies that drop brains that can be used to craft a brain in a jar.

In tainted areas most vanilla, peaceful, neutral or hostile mobs will become a more powerful tainted hostile mob.