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.
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 !