So Credence ends up with Newt all cured and shit but he's become desperately attached to the safety of the suitcase and doesn't want to leave. Newt tries to coax him out but when he promises to do anything Newt gives him a hurt medium sized creature to care for to help him gain confidence. Credence takes this very seriously and when the beast gets horny he lets if fuck him thinking it's part of his duty. For whatever reason other male beasts start taking an interest in Credence and he ends up there willing eager slut.

Newt could have had good intentions and not realize what's happening or he gave that beast knowing what would happen and deciding to use Credence for relic for the animals.