The Pegomastax is a very special tame. If u put something in your last hotbar slot it will steal that instead of anything else. So put ca. 100 Mejoberries in your last slot and any Pego under lv 50 will be instant tamed.
So, I don't know why, but me and my sister tamed one when it got stuck under our triceratops. I think it was stuck, but we just slapped it and that tamed it. The thing is, I had spent fifteen minutes before hunting down one with my bow and hatchet that had stolen my sheild. I really hate them in the wild. They suck.
These are only found in the dungeons in mobile. You can resurrect them once you have their implant on a obelisk with a built in trap or on a revival platform with a trap and a solid amount of amber. Keep in mind, these are passive tames. Like so people can see.