"These red-robed Chosen-in-training have started their transformation into what you once were.... The Chosen. Killing them is only the first step, as they transform into the powerful Beast, whose stomp can open up a world of pain."--Blood Manual
Its conception likely came from a dropped feature from Blood. Originally, Caleb was to be able to tap into powers of bloodlust, which would enable him to transform into a giant beast. This power would activate when having taken too much damage, or causing a lot of it. There were also plans to have each of the Chosen be a playable character, who would also be able to use the powers of bloodlust, each transforming into a different monster. These would likely become the bosses of Blood (Cheogh, Shial, and Cerberus).
The Beast starts out as a Priest - a red-robed with golden belt cultist who wields a shotgun. The storyline states that these are "Chosen-in-training", transforming themselves into what Caleb and the others once were. They wait in a stonehenge-like arena for Caleb to come. These enemies fight like normal Cabalists, but with a higher shot rate and almost triple vitality. When they are killed they change instantly into The Beast.
When a Priest is killed, it turns into the Beast - a creature resembling a wolf, twice the size of man, with stone-tough skin and blood red eyes. They attack by slashing and stomping, which creates a damaging quake and also freezes the player. This stomp can damage the player through armour and can even affect him if he's not on the ground.