Nightmares are tricky. First, nightmares, as such, are not indicative of serious neurological or behavioral problem, so there is no need to be alarmed on that account. Dogs and bulls, in the real world, are for sure potentially quite dangerous. The "dream image" of a four legged animal may be a "place holder" representing something else that we do not see explicitly or clearly in the dream. I would expect that to be another person, but revealing the "trouble maker" and resolving the situation may be easier said than done!