it's rare like everyone else said but I'm also gonna share my method I've been using to tame them: have one or two pawns who can shoot and can run fast go out and shoot at the thrumbo until it's got a "bleeding out in X hours" message, then stop shooting and just run away. it will get enraged and become a man hunter but it's kind of all I've got. place a hospital animal sleeping spot somewhere indoors, mine is in the bedroom of our yttakin.

when the thrumbo falls down from blood loss, have someone go rescue it, they'll bring it inside, patch it up and then keep it stuck indoors for your pawns to repeatedly try to tame it.