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, various Transformers were born with distinctive beast modes, being based on Onyx's template. These ancient Cybertronians would reside in Bestia, one of the Thirteen Realms.
During the War of the Primes, many of the bestial Cybertronians were called to arms and allied with the remaining Primes and their realms against their betraying brethren. Some of them were big and powerful, which helped ensure their victory against their enemies; among them were the Dynobots, who were the first to have saurian-based beast modes. 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 Animatron, and scanned the fauna of a nearby planet, Earth, for new beast modes for the planet's inhabitants.