Henry McCoy was a genius. He became one of Apocalypse's leading scientists and spent his time working on expanding genetic mutation alongside his mentor, Nathan Essex, the man known as Sinister. McCoy was responsible for creating the process needed to build the gene-spliced Infinite army used by Apocalypse to take over much of the world during the Age of Apocalypse. He would normally take those mutants deemed unfit for survival by Apocalypse and perform gruesome experiments, attempting to create an unbeatable army of enhanced mutants for his master.
McCoy also conducted experiments on himself attempting to make himself the perfect mutant. Instead, he turned himself into a monster known throughout the Breeding Pens as the Dark Beast. During the final days of Apocalypse, following the mutiny by Sinister, McCoy became even more radical in his experiments. Without Apocalypse's Horseman looking over his shoulder, the Dark Beast was free to do as he wished. He burned through thousands of mutants looking for the ideal combination of genetic power.
During the final hours of Apocalypse's rule, Jean Grey and Prelate Scott Summers freed many of the mutants the Dark Beast kept in his Breeding Pens. These freed mutants rebelled, and the Dark Beast, realizing many of his captives would be looking for revenge, went into hiding. He was found by the X-Men when they teleported into Apocalypse's stronghold. The Dark Beast, knowing Apocalypse's kingdom was falling apart, teleported himself into the M'Kraan Crystal, taking his chances on the millions of realities that lay inside of it.
The Dark Beast traveled back in time and ended up in the main Marvel Universe's reality at the same moment the boy known as Legion was attempting to murder Magneto. A young Emma Frost discovered him and they worked together briefly, but the two parted ways. It is believed he is responsible for the creation of the Morlocks and Gene Nation, as well as many others.
At one point, the Dark Beast kidnapped the good Hank McCoy from the X-Men and entrapped him. The good Beast eventually escaped and returned to the X-Men. The Dark Beast fled. He recently resurfaced in the ruins of Genosha, apparently having joined Charles Xavier's efforts to rebuild the nation.