Drawing a character as a pony some of the time does not make that character a pony when you draw them as other species. Tag the pics with not pony related and move on.
If the OC is “popular”, like, say Milky Way, Ticket, Little Pip, Nyx, Snowdrop or what have you, then sure. If it’s an OC that has all of 3 or so images on the booru and practically no one could recognize, then no.
Is there any sort of standard of recognizability attached? Like if someone is a blonde female human, but she has no wings, no horn, no tail, no cutie mark, and a different hairstyle, are there any grounds for still calling it Ticket?
@Background Pony
Unless she’s doing anything Ticket would be doing (whatever it is she does) or the image contains something else that would clearly tell an average viewer (that knows about Ticket) that the character is Ticket, then no.
We already do delete images that just happen to be “blonde girl with green eyes” that OP says are Applejack but are actually completely unrelated, so an image no one would recognize as being character X would get deleted just as well.