Future expansions.

Well, unicorns were originally creatures of Sylvan, and then the developers decided to diversify their appearance and create something like that. And phoenixes were created from the very beginning as elementals of Ash.

By the way, regarding phoenixes in Academy, that's how it was in Clash of Heroes.

Very insightful observation, in my opinion. Some elements from that game were already used in Heroes 6, and they might borrow more ideas.