The fifth post here has a pretty thorough list of what is/isn't mutant or allowed period.
Posted: Mon Nov 22, 2010 12:06 am
Variations of a breed feature were once mutant traits, but no longer - the reason being it seemed ludicrously silly to have, say, a custom drawn uni tail classify your horse as a mutant. Think how flutter wings have always worked - so long as the basic design matches, you're good to go.
(Exception to this is large wings (other than flutters) because they make colorists wail and beat their chests when requested.)