Moters appear as two Attack Flies joined together in The Binding of Isaac.
They behave similarly to regular Attack Flies, but split into two Attack Flies upon being killed. Their split is very wide and unpredictable and can easily shave off half a heart. It is recommended to kill faster flies and other small monsters before it as the two flies it spawns are actually more dangerous in many ways. Running between them just as they split is also a very good way to dodge.
If Isaac has Skatole, killing a Moter will not make them split into two Attack Flies.