NEVER UNDERESTIMATE THESE GUYS, they are able of dispatching your tamed velonasaur in seconds because unlike ridden velonasaurs. They can fire at a longer and more rapid rate. If your velonasaur is un-mounted, then it may very well be able to kill the wild velonasaur!
If you want to fight or knock one out, get up close and hope that it will then opt to use its melee rather than it's spines, as the latter will shred you in a matter of seconds. Trapping it with a bola is also a good method as it will be unable to turn its body to fire at you, but still be wary of it's AOE where it shakes and shoots spines in a small radius around it.
So I was trying to tame it on a tapejara... When it is in turrent mode, it didn't hurt my tapejara. So maybe it's a glitch/bug but it won't hurt your tapejara. But I would shoot it off my tapejara obviously, then hop back on my tapejara and wait for the torrents to be over. But attacking a velonasaur while on a tapejara doesn't do damage unless the velonasaur hits it with its claws.