A Beast mode is a term used in Transformer biology to describe a Transformer's alternate mode when it is in a form of an animal.
Upon the first activation of the Well of All Sparks, Onyx used his template, as well as the powers of his Triptych Mask to scan life forms through the universe, so that various Transformers were born with distinctive beast modes. These ancient Cybertronians revered as one of the greatest Primes and eschewed the vehicular alternate modes of other Transformers, preferring to wear the animalistic forms exemplified by their spiritual progenitor. He eventually formed his own tribe of followers who followed in his teachings.
During the War of the Primes, many of the bestial Cybertronians were called to arms and allied with the remaining Primes and their followers against their betraying brethren. Some of them were big and powerful, which helped ensure their victory against their enemies. Unfortunately, this wasn't enough to stop the infection of the Well with dark energon, resulting in newborn and deceased Cybertronians turning into powerful, monstrous creatures. Many bestial Cybertronians were killed by these creatures, forcing most of the remaining forces to flee.
After the end of the war, Onyx led a colonization expedition aboard the titan Ascella to create a haven for bestial Transformers away from the devastation of Cybertron. He eventually founded the colony Eukaris, and scanned the fauna of a nearby planet, Earth, for new beast modes for the planet's inhabitants.