What about a lizard AI that would help the player by attacking bugs, and heal when it killed one?
Or maybe a BIG-ASS boss spider that spits venom, runs fast, leaps high, has a lot of health and/or defence, and has a really climactic death? I'd give it a catchy name, like Big Mama/Hulking Mama.
I was also thinking about a parasite (possibly a flea?) that would latch onto you and drain your health.