Question: what are the sewer creatures in mockingjay?
Answer: They're “mutts” or (in the books) “muttations,” mutated animals created by the Capitol to serve as weapons. You might recall other types of mutts from the first Hunger Games movie: During the Hunger Games, the gamemakers send tracker jackers (huge, venomous wasps) and wolf mutts (huge, vicious dogs) into the arena.