How tough do you want it?
How about a flock of vampire bats? Hungry, blood drinking, flying things, easy to kill individually but you have to fight them as a swarm. Everyone gets multiple opponent penalties! Non magical but magic is not too effective against them unless the caster doesn't have any qualms about his/her magic also affecting the heroes that the bats are sucking blood from.
Enjoy!
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> Now to you esteemed gentlemen : I need an idea for an opponent / monster found on the wreck of a waertagi dragonship on the Edrelynn islands. It already looks like the "Nostromo", with grues in the holds and waertagi ghosts fluttering around, but I need a flyer which nests on the ship's bridge, and takes off from there to go look for food. I had fought about a chaotic wyrmspawn of the Mother of Monsters, but then why wouldn't it have eaten the whole neighboring waertagi colony already ?