Hawkbane does not show up on NPCScan Overlay. Silverdragon will pick him up and he can be found with NPCScan if you add him manually. His NPCID is 21724.
You can also use this macro to help find and tame him:
/cast Tame Beast
I'm not sure as to his spawn timer, but I can safely say it's not a phenomenally quick one. I had to come back to the area several times before I finally found him.