I was fishing in the upper left area of Un'Goro, where the river ends in a 'duck head' of pond with a little island in the center. I kept an eye on Track Beasts in case the Pterrordax I killed respawned...and here comes a red dot! Fast! Turn around, see it, scream and flail for a bit before feigning death.

Then I had to watch as it flattened my poor little raptor. :( So Devilsaur goes just past the pond, turns around, heads back south. Once he's gone I rez my pet, feed him and make comforting noises, and return to fishing. Before my lure has run out he's BACK! Feign again, watch pet die again. Find another fishing hole.

In conclusion; if you're waiting for a Devilsaur, consider waiting just above the duck head in the north west.