Leech (different/more thorough than other suggestions)

A typical leech. They attack you like a silverfish or mite, but they actively heal every time they attack. They are almost completely black. They could drop an item that could be used to make a potion of leeching or an enchantment that lets you steal the life of other things. Or they could not drop anything. Leeches are neutral-aggressive above water, only attacking if you get within a 1-2 block radius, and aggressive in water, actively seeking you out like a drowned. They're scared of axolotls. I also propose that they're repelled slightly by firefly bushes, unless aggroed. I'd prefer if they move in a slinky manner rather than a slithery manner like the other two mites.