I’ve owned thoroughbreds in my life (ok, my wife owned them), so when I saw this card, I thought it should come with a matching treachery called “Plastic Bag”. (When plastic bag is revealed, attached character takes one point of damage. Shuffle Tireless Thoroughbred back in deck.)
Fortunately for horse folk in Tolkien’s world, there are no plastic bags in Middle Earth. The art on the card seems to portray a Rohan rider and those guys train their horses to withstand howling orcs with sharp objects. Rohan horses wouldn’t be scared of a plastic bag. Probably.
Tireless Thoroughbred is a niche card for multiplayer. I can’t see using it in solo unless you really, really want to put an attachment that’s only for ranged/sentinel on a character without those traits. Using it in multiplayer makes sense if one is playing some kind of combat based helper deck but it’s expensive for what it is and can only attach to warrior characters. And there are already decent warrior characters with ranged or sentinel. I think adding a weapon or armor to a character with those traits would be better but for some of you tricksy deck builders it adds a little more flexibility.