It all began when Mitron wished to become friends with the Warrior of Light. He was an idiot, and thought that bringing back their ally, Haurchefant, would be a good way to do so!
Without telling them.
And without studying the Source.
Thus, they empowered the broken body with dragon blood, and left to let Haurchefant live! Planning to come back to get lots of praise!
It has not been good for Haurchefant. He has been stuck between dragon and human forms, both fighting for control. He still feels anger and injustice towards the world, even if the War is over. A relic, of a time now fading. He is also disabled - his body aching too much to strike at anybody.
In the time of Endwalker, he followed the Warrior of Light, stuck in his full dragon form. He could not turn back, for he was filled with his feelings of anger and bitterness, and could not focus enough to turn back.
And thus, he got to see Ultima Thule. He got to stand in the Dragonstar, the origin of his enemy.
And Haurchefant wept. He wept, and could not move on. Even moreso as his fellow Ishgardian turned to the wind, the breeze calming him. By the time everybody came back... he was mostly Elezen again.