Couple of things,
Phoenix was made tameable in Publish 97 (april 27, 2017). It was changed from the red eagle graphic to the greater phoenix graphic in Pub 97 bug fixes (06/02/2017). The reason for the change was due to pvpers crying about their overcapped dex (this was fixed in Pub 97 bug fixes too, with a 190 effective dex cap being put on pets) and their small size making it hard to target. The greater phoenix (which aren't tameable) and their accompanying graphic were released in Publish 90. So they already had a graphic for the tameable phoenix they could use.
With their small team size making new tameables or new tameable models would take artists off other projects, which is why we get limited new mob models at this time.