I wanted to use my favorite little tyrant, Moros Intrepidus, and this seems like the perfect opportunity to do so! These guys were found in a mountain formation, so they would live in centers highest peaks. They were about shoulder height on an average person, so they aren't too large. They would have been scavengers, eating off of others kills. That leads to the idea:
TAMING:
Moros would naturally stay a distance from large predators (yours or wild) and wait for a kill to be made. Then they would charge in and try to steal it! If its a wild creature, they would fight, if its you and your lvl 400 rex, then you can tame it! By feeding the little fella a bunch of corpses, kind of like the fjord hawk, you can tame them. These corpses need to be fresh too! so you cant pre prepare corpses for the thing, it wants to steal em from you, not have them handed to it! Once you've fed it enough you can take it home as yours!
WILD:
As previously stated they hover around large predators in order to steal their kills. They are slightly territorial, so don't get to close. At the same time they will run from larger dinos such as a yuty if its too close for comfort. They would most commonly be found around large mountain dinos like yutys, trex, and even up to a giga, which could have 3-5 of em following.
TAMED:
These little dudes actually pack quite a punch, maybe due to them being very closely related to T-Dominus? (arks rex species) They have a weak bleed and heal off corpses like argentavis. Now the fun part! If you feed them certain trophy parts, they get new abilities. These would be permanent, until replaced by a different one. This could be things like feeding it wyvern talons and it could cough out small flames/bolts/poison-puffs. Or maybe feeding them an amargasaur quills and it gaining the spike launch ability (marginally weaker). Many other possibilities, like maybe a wooly rhino for a charge attack?
SIDE NOTES:
I would size em up to be carno sized, to fit the role better.
They would be decently hard to tame, and are allowed in boss arenas unless fead a giga/charcar. (hehehehe)
The wiki link:Moros intrepidus - Wikipedia