The thing that was in trevor's mouth

When Trevor drowned an eel got lodged in his throat and died there. Nothing more, nothing less.

- Scarecrow
(www.cenobite.com)

During his nightmares we saw a scene, when a much bigger and more disgusting creature is coming out of his mouth.

This was an exaggarated imagination of the real event, so this final scene (when the coroner pulls out the fish) explains why we saw it earlier.