He didn't charm it, if you do mesmer class you get a mission to kill a bull, the owner of the bull demands a devour egg for compensation to raise as a pet to guard his farm otherwise he will press charges. So Old Mac actually raised his devour from youth.
I think devours could be cool pets though, the thing is devours are a race of enemies all to themselves, with 3 different variations, so I don't think devours fit the bill. As for spiders, bunnies and other animals just walking around, I think it would be cool, but I don't think it's planned, and they don't realy make good attack pets......... I mean Bunny?, that just silly. Too bad you can't tame the mad bull

he has hella armor!