We ended up accidentally inventing a number of monsters yesterday. Here are the ones I remember:
Storks - These are stirge-orcs. Basically orcs, but they have stirge proboscises and suck blood, attacking with an axe swing followed by a close-combat eye poke to suck out your precious bodily fluids.
Orges - flying eight-legged orcs. I'm not sure if they suck blood, but they sure do suck.
Strolls - Stirge-trolls. It's not clear if these are blood-sucking trolls, eight-legged flying blood sucking trolls, or stirges that regenerate and say "Hloo, hloo!" a lot. Maybe all three.
Turns out that, like trolls and rust monsters, trolls and stirges are natural allies. They just regenerate all the sucked-out blood. It's worth noting that any two monsters encountered together in my games, or in the same dungeon as each other, are declared "natural allies." Oddly, monsters that have the most affinity are "natural enemies." Earth elementals and gargoyles? Clearly natural enemies!
It's Drystian Naturalism.