Time to go back to the Age of Apocalypse.
Dr. Henry McCoy constantly experiments
on himself to enhance his superhuman powers.
Yes he does, and yet somehow he never unlocked his "totally natural and innate" secondary mutation? Maybe he did discover that timebomb waiting within his own genetic structure and merely opted to side-step it, recognizing that suddenly developing paws would be a hindrance to the continuation of his genetic experimentation (the experiments he's doing on others, not on himself - one mustn't impede the progress of scientific cruelty, after all!).
Logically, this figure uses many of the same molds as Series 5's Beast. That means he's got the large upper body and the detailed fur. However, every bit of the figure between the ankles and the stomach are new, because while Earth-616 Beast just wore short trunks, Earth-295 Beast wears metal pants. Yeah, it's a weird fashion choice, but it's probably inspired by his mentor, Mr. Sinister. Giving us those banded metal trousers required some new sculpting, not just paint. The original AoA art showed him with quills poking ot of the fur on his back, and while those aren't here, there was plenty of art over the years that ignored those, so we're not counting it as an error.
Same goes for the hair. Originally, Dark Beast just had a head of long hair, like anyone could grow; but eventually artists began drawing him with a larger version of the "Blueverine" hair normal Beast had. If nothing else, that helps make clear that this is Hank McCoy, and not just some random fur monster. His advanced mutation has also left him with large fangs sticking up from his lower jaw, and even larger ears than usual (the earrings in them are probably not part of it).
Unlike the regular Beast, Age of Apocalypse Beast never had his fur change color: though they were both grey at the start, "our" Hank first turned black (in Amazing Adventures #15) and then shifted
to blue sometime before Avengers Annual #6; evil Hank has been grey all along. Well, except for the period where he turned himself blue to impersonate 616 Beast. The color here is a great shade, and his pants get a metallic sheen. His white teeth really stand out against the fur, though his yellow eyes are so small they tend to get lost in the shadows, and the red wraps around the two... pigtails? that dangle over his shoulders are fairly dark.
ToyBiz released a Dark Beast in 1996, but it barely moved. Even with the new legs, this one has all the same articulation as the Series 5 figure: head, neck, pecs, shoulders, biceps, elbows, wrists, chest, torso, hips, thighs, knees, ankles, and toes. He's also got the same four swappable hands (one fist, one flat, two curled).
He doesn't really get a "piece" of the Build-A-Figure as much as he gets an accessory for it: Sugar Man's big square hammer. Which says "SUGAR" on the side. Weird character.
It's kind of strange to get an entire series of "Age of Apocalypse" Marvel Legends, but if you're going to do it, Dark Beast is a smart choice. He's a major character, and lets Hasbro get some more mileage out of some fairly unique molds.