If one is encountered near water, you can simply lure it out to sea with a flyer and it will swiftly drown. Given the water is deep enough. If encountered inland not near water. Pray to the overseers you have something fast enough to out run it. Then promptly die because they're as fast as a mad Hispanic mother with a shoe. Not as deadly as one though.
If you NEED to kill a wild one with a mount, try to trap it in a taming cage then use an Allo/Carno/Thylo bleed. Or if you have a juiced up thylo, you can run in a circle and bite its ankles, but this is hard and risky to do.