I don't think this is as powerful as you're selling it with the mythic icon and legendary status.

It kinda reminds me of Mikaeus, The Unhallowed except that it only affects one creature type and immediately stops itself from being abused.

I demoted it to rare. It is pretty niche in most sets and situations.

I also added haste to it and to those it ignites.

I wonder whether people in general will know whether the returned creature is still a Bird once it becomes a Phoenix. It's not a common wording this way.

It's not still a bird, though. See the rulings on Contaminated Ground.

You don't need to explain that to me. You need to consider whether you shouldn't word your card in a way that makes that abundantly clear.

Lands subtypes often use this ability and players are going to recognize the interaction because basic land subtypes come with rules. Creature types are far mor often given "in addition to its other types" and the human brain is nothing if not a system of shortcuts that will miss the difference.

Reminder text would fix that up if, in testing, it wasn't apparent.

I still just don't get this card. Is it a bird tribal card? A phoenix tribal card? Since it reanimates immediately, is it an aristocrats card?

Not to mention that birds =/= phoenixes. It's awfully nerdy for me to say but this is like turning Horses into Unicorns or Drakes into Dragons. They're explicitly different species that just happen to look similar. You can't just light a bird on fire, say the magic word and bam, it's a phoenix. Heck, I'd be less mad about this if it affected every creature with flying and it retained it's other types (but then specified that it only reanimates non-phoenixes)

Well; with this in play - all birds can become pheonices. I have no problem with the flavour; it is wonderful. "Where does a pheonix come from? Well, when a bird dies; sometimes Sunwing burns its body to ash - and the bird rises; no longer a bird; but as a pheonix".

So yes; explicitly; it IS like turning horses into unicorns. "Stonehoof the horner" would be a wonderful extension to start a cycle here.

Personally, my only issue is 4/4 flying-haste for 4 means its (slightly odd) tribal thing won't actually get much of a look in. And that the 'nixes it makes are only half-nix, since they can't also re-animate unless something else does that for them.

... And this does really painful rules-things if the opponent has one too. So although "I kill your bird, steal their corpse, set them on fire, and fling them back in your face" is hilarious - it's also such an edge case, that I'd suggest making this "a Bird you control" instead.