Eren has always hated being caged in. He doesn't care about impossible odds. He does still feel fear; it's a motivating force for him and always has been. But his fear is not so specific. He's not afraid of monsters. (Joke's on him, he'll learn about the Darkwalker first-hand someday.)

He almost says me in answer to Billy's question, but that would be more explanation than he cares to go into. Anyway, he wasn't always a monster (well…he was, but that's complicated).

"…Titans," he finally says, aware that it's unlikely that Billy will have heard of them. No one else has so far. "Giant monsters that eat people."