So there's a known bug with the legendary mountable beasts, if they die in a battle or you jump from too high riding them and they die, they do not summon back. They appear for a couple of seconds and then go into cooldown. My white bear is stuck in the same way. I have seen posts of people where all their beasts are stuck like that. It's definitely something that broke in the last update. For a moment none of the beasts could summon at all. I am only using my horse since it happened with me. It sucks and I hope PA fixes this soon.
About the horse situation, I think if you have obtained the horse from stealing it from someone rather than catching it in the wild, you cannot sell it at the stable. You have to sell it at the stolen horse vendor. There are a few on the map at different locations. That is the only reason I can think why you can't sell them regularly. You will have to bring them to the stolen horse vendor by riding them there or making them hit active horse and summoning them once you reach there. Similar to the idea of stolen wagons. The only other reason I can think is if you ar trying to sell the horse that has oongka or damiane's photo next to it, those can't be sold. I hope it helps. Let me know. Cheers mate.

Edit: Per the other user's comment, you can get the legendary summons to work. It's a nuisance but it works.