You can't call him the Doom Slayer if he doesn't have anything to slay.
Even if you've never played Doom, one of the most important videogames to ever exist, you're still likely to recognize some of its enemies. The Imp is just a brown humanoid with some silver spikes sticking out of it, the Cacodemon is a flying meatball, but the uncreatively named "Demon" (isn't everything a demon?) is a big pink slab of muscle with a head as large as their entire torso. Due to their bad name and distinctive coloring, fans called them "Pinkies," and the name has been officially adopted by the dev team.
The Pinky has changed a bit over the years, even ignoring the period (Doom 3) where they were smooth little slugs with cybernetic wheels instead of legs. (Fun fact: the original Pinky's legs were modeled on a Jurassic Park toy.) Doom 2016 made Pinkies ambulatory again, but also covered them with armor on the front and turned them into a bullfight boss. They also gave it a tail that might have been inspired by the JP dinosaur toy.
Despite its folded-in-half posture, the Pinky is surprisingly small. Mystery Minis are approximately 3" tall, but this one is only like 2". Aren't Pinkies supposed to be taller than humans? That was always the impression they gave. If the Fallout Sentry Bot could fit in a standard Mystery Mini box, so could a correctly sized Pinky. One that's huge. And has huge guts. Bursting-hot blood-packed freakopathic creature guts.
The armor is darker than the skin parts, and the many, many spikes on its body (and teeth, and horns) are pale yellow, way lighter than the yellow of the eyes. Between the head and the tail, he's very well balanced, standing securely on his stumpy little feet.
Pinky wasn't in the normal Best of Bethesda cases, only the GameStop ones. And he was only available in a 1:24 ratio, meaning if your local store only got one case, there's a 50/50 chance Pinky was never there.