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I read about it in a PDF that details the Beast Taming abilities. Also mentioned in the pdf if Zone Rider and Beast Whisperer classes or skills. 

A Bitterbeast is a monster that can be tamed and be used as a mount (Mutant Year Zero: Core Rulebook p.178).

For some reason, I have always ‘seen’ & described a Bitterbeast as a giant, acid-spitting salamander.

For some reason, I have always ‘seen’ & described a Bitterbeast as a giant, acid-spitting salamander.

That is the description (including art) that is in the corebook and that also is in all the earlier versions of mutant throughout the years. There was some new art in Die, Meat-Eater, Die that made them look like a smaller versions of a t-rex. Regardless of what description you like, the bitterbeast can be used as a mount, if tamed.