I don’t know if this is a bug or not but during Mount Mania, the Maldraxxian Corpsefly was called but I couldn’t mount up because the character I was on was not a member of the Necrolords. Shouldn’t this restriction be removed, at least for Mount Mania?
By default, you do need to be in the Necrolord covenant to use it, but you can remove the covenant restrictions for your account by reaching renown 80 with the Necrolords on at least one character.
Profession mounts which require that profession to use such as flying carpets and flying machines can appear as well, so the way the contest was designed doesn’t care about any restriction (or the rarity of any mount either, for that matter; see: Gladiator mounts appearing as challenges).