When you see one, put any kinds of berries in your last inventory slot and walk up to them slowly. They will take the berries and it will add to their bar. Just keep doing that over and over
-bboss505 (pls up-vote
Make a taming pen. Put mejo berries in your last slot and let the pega steal it. After it steals the berries use a whip to take it out of its hands and it will steal from you sooner. Only way to tame a 150 it seems since its taming bar goes down while it holds the berries.
Pegos can be held in the beak of an argent (press c on pc to pick them up) and they will stay in there when you dismount. Holding the pego and then running and jumping at the argents face will let it attack you and take some food. It's a bit janky but I tamed a 150 this way when my taming pen was full
I have discovered the way to easy tame these things!! What you need to do is trap the pegomastax in a closed area and have two tribe members with a bunch of mejo berrys ( make sure you always have a stack of 100 in the last slot of your hot bar) make sure both of the tribe members are standing next to each other and when the pego attacks ( robs ) you it will hit both of you at once making the taming a lot quicker and easier !! You're welcome !