Grimlock ♥ Ultra Magnus

"...frothing, growling Onyx..."

Sometimes there are very brief descriptions that get to the core of the Primes.   They are thirteen different aspects of Primus - we're still with the Thirteen, although with Solus Prime gone, there are only twelve left - so we're only dealing with them.  

Onyx was the mystical/animal side of Primus, and in his rage and grief, 'frothing, growling' just gets to the heart of him.   (Well, there's more, like the description of him attacking another Prime who might know more about Solus' death, and so forth, but at this particular point in time, you just needed those two perfect words to describe him at the moment.)