Grubthor can be found with NPCScan. You will need to add it, his NPCScan ID is 14477.
You can also use this macro to help find and tame him:
/cast Tame Beast
I'm not sure as to what his spawn timer is. I had to check the area several times before I found him. With so few people questing in Silithus though these days he's a lot less likely to be killed by someone questing in the area.