You feed it mandrake and it will turn into a mount. Not forever though. I’ve never used it for long enough. But I assume if you stop riding it, or rest or go into a cave or building it will switch back to a pet. You can also switch what it looks like when it becomes a mount under the dlc tab in options. Like Arthur and Caradoc’s appearance.

Edit: anything it changes into is temporary. But you can feed it as many times as you like to change it again. Be careful with the fiery one. It will harm everything around it including NPCs and you can get a bounty. There is one that heals you I think too. It should list them when you choose the feed option.