Lesson horse I've been riding this winter does that. He's a whole lot easier to bend, turn, leg yield in one direction than the other...and it also shows when he canters. In one direction, it's a proper three-beat canter (and lope, depending on how much leg I've laid on). In the other, it's canter-ish, but it's a weird four-beat gait that is pretty herky-jerky to ride. My trainer thinks it's due to the same lop-sided development that makes it harder to bend him.
I've found that laying on a leg aid and asking for more speed helps. About 50% of the time this will cause him to smooth out the canter and hit it at a proper 3-beats. The other 50% of the time I just get a faster choppy canter-ish thing. Even if I get him to smooth it out, he still wants all the time to drop back into a trot in that direction.
Now, this horse has a good excuse, because his usual rider is handicapped and has some major balance and strength issues on one side, and that corresponds with the horse's one-sidedness. Who knows if he'd be like that anyway and is just a better match for his usual rider because of it, though?