Bull was, by a fair margin, the largest toy released in the Convertors line. He got a whole assortment to himself, that of the Jumbo Defenders. Or Jumbo Defender, singular. I've no idea what line he originated from - the only markings are 'MADE IN JAPAN'. I'm guessing he was originally called Buffalo at any rate, there are a couple of subtle clues on the figure to that effect... It says it all for Select that they didn't bother modifying the sticker sheet, or just call the thing Buffalo...
The only non-Convertors example I've ever seen is a bootleg-looking thing which basically had the Convertors packaging with the trademarks replaced. Select didn't make anything for Convertors as far as I can tell, while the thing's far too well made to be something Mark came up with. Answers on a postcard, please...
Bull's alternate mode is a 5.5" long bulldozer. It looks a little unusual, having no cab cover - well, that's probably not particularly unusual I suppose, but my knowledge of the things basically begins and ends with Bonecrusher, Dozer, Dozer CG et al. The orange is a nice colour, though it's a shame the blades aren't grey or silver. Despite the realistic-looking black hydraulic-type parts on these, the blades don't actually move on the front of the arms, though the whole arm section can move up. The tracks are solid metal (Bull moves through four coaster wheels underneath) and while there are some nice stickers, it's very mundane for such a large figure.
The transformation is unusual, being partially spring-loaded... What's odd is that most gimmicks like this tend to be fully semiautomatic (yes, that looks clumsy, but 'fully automatic' would be incorrect, I think - it's not fully because you have to do something), the only exception I've encountered being the concept-taken-too-far awfulness of Eagle Winner. I guess you could argue it's the forerunner of Automorph. For Bull, you have to put the legs in place manually from the tracks and blades, then pressing a button on the remaining section causes the head and arms to pop out. The robot is an odd looking thing - a perfectly conventional top half with decent robot mode features, and then weirdness from the waist down with strange legs. It's a rather poor design, as the screws on the blades just don't have the rigidity to properly support the robot (there are no notches or anything to lock them in place). And it looks weird.
It's a shame, as he has more personality than a lot of the line, and the upper robot design is sound. You just can't trust him to stand up...