I recommend making a decent sized closer off area before hand, make it so an argavantis can fit.

Get a wooly rhino, or any other forager. Find MANY rockcarrots. They want a variety of berries majoring rockcarrots.

Then search on an arg. It could take hours to find a single one. Pick it up and bring it to the closed area.

Put all the berries in your hot bar. It will make grabbing the berries easier. Click the saddle and start rapidly giving the required berry. Repeat many times.

That’s how you get your own equus! Good luck! :D