One of my first mods was to add a trainable tags to the water buffalo, as well as to make them a carnivore. This, I have to say, is one of the most useful things I've done on Dwarf Fortress. They are pretty much no longer water buffaloes, now they are blood thirsty killers. I'm calling them the "Mad Cows of Visechambers (my fort's name)." Every single thief, snatcher, hostile has met their ends to something along these lines: "... gores The Kobold Thief in the head with his/her left/right horn, brusing the muscle, jamming the skull through the brain and tearing apart the brain."
I have yet to face a goblin siege, not sure why (I have 134 dwarves, ~72 k wealth, though I can't penetrate the aquifers, I just can't figure it out...). I'm a bit anxious; I want to see how these war beasts do in larger scale battles. I could have made them only war trainable because that is the only aspect I use with them. I try to pair two (male+female) Mad Cows with at least one, hopefully two, war dogs. The dogs get there first and hold the enemy, then the Mad Cows come in and obliterate them with one blow. I embarked with a 3+1 breeding group of Mad Cows and a 3+1 breeding group of dogs. I also embarked with a dwarf with high animal training skills. Meat, Bone crafts, milk galore. I have a feeling that there is something big coming...