I agree, water buffalo in Star Wars? How about a Dewback in its place? Or were you perhaps using the military lingo for "water buffalo"? (it's a large trailer with a tank full of water).
I have to agree the guards would never let something pass just because it happens to look absurd. There has to be another reason.
Another reason? It's a gift from the villagers the bandits have been raiding. It's not a real Dewback, but a large fake one on wheels made from scrap in the village. Its pulled up and dropped off at the front gate by a landspeeder that drives away leaving a note to the bandit/warden that it is a peace offering and bribe to no longer attack the village........You see where this is going.