User blog comment:Paul.rea/Teen Wolf News 020117/@comment-4554525-20170202073240/@comment-4091815-20170202074146

The first is a plot question and if he continues on the show that could be a key in future. He won't answer those types of questions.

All the creatures you mention don't have their own free will. Without free will, they can't join a pact because they can't decide to do so. All are forces either conjured or controlled by others or are forces of nature outside control (Ghost Riders) - none of them can be part of a pack.

This doesn't apply to chimera but they're not making any more of those.