The opposite of a docile tard, a bull tard is person who is both retarded* and extremely aggressive. Often possesses "tard strength." This person would rather fight you than talk it out; would rather use violence than intellect; would rather use brute force than finesse.
*Does not necessarily have to be actually retarded...simple dumbasses can also be tards, and by extension, bull tards.
Friend: Dude, your little brother just threw his bowling ball halfway down the lane and put a dent in our lanes!
Me: Yeah, sorry...he's a bull tard. He breaks things.