Free flea concept: a puppeteer or brainwasher.
It latches onto other pokemon and controls them. You can’t catch them like other pokemon; instead, you can sometimes accidentally catch one when you’re actually trying to catch another pokemon. That is, it would seem like a normal encounter with some other pokemon, but when you try to catch it, you instead catch the flea that is on that pokemon. If successful, you would still have a chance to catch the freed host.
After catching the flea, it still requires a host. There is room for creativity here. My first inclination is to use a modified version of the Substitute doll. Maybe you could have a choice of various pokemon dolls which would constitute forms for the flea to take, sort of like Rotom.
The brainwasher route is obvious. The puppeteer one could be more interesting. Maybe the flea has a body like a ball of yarn, and it has sewing-needle limbs.
Anyway, I think this is a good idea, but I am sick of trying to draw fleas and I didn’t want to cop out and only submit a drawing of a crazed Substitute. ;)