Humans

the humans are an unknowalbe species that may not even exist

while we never actually see them, on the museum door in worms battlegrounds it says "humans only".

but, the most hints come from the worms WMD. the first is when the narrator says "or crawl if thats what you worms do!" suggesting that they are a human. in the brimstone battleground, we can see a statue of a human foot as well as a giant high-heeled boot on top of a shop.

some worms might be friends with humans because if an enemy worm enters a building with a worm with the "snobby lady" speech bank she will say "please take your shoues off when entering the house!", but worms dont have feet so they cant wear shoes