This is perhaps one of the most bizarre Transformers ever made, with a pretty unusual history to match. Although the toy's most prominent mode is a head, this is not really a Headmaster, since there's no toy it becomes the head of... at least, not unless you get creative....
Obviously, I need to backtrack a bit. This toy finds its origins in the second season of the Beast Machines cartoon. At the end of the first season, both of the leaders, Optimus Primal and Megatron, destroyed themselves in a no holds barred battle. Of course, this situation didn't last long. Optimus Primal was already back to normal form by the end of the first episode of the second season. Megatron didn't have it quite so easy. In his quest to eliminate all organic matter from his body (obtained in the previous series: Beast Wars), Megatron had created a giant ship in the shape of a head that was intended to house his spark. Eventually, Megatron's spark did make it there, but that took a couple of extra episodes to happen.
Hasbro apparently decided that this giant ship that looked like a head needed to have its own toy. But they took some time in doing it. In fact, the toy didn't come out until the Robots in Disguise line, by which time the Beast Era was a thing of the past, so the bio put on the toy made it clear that this toy represented not the Megatron of the Beast Era, but the Megatron of the Robots in Disguise cartoon, who was an entirely different character. And, in an attempt to tie the toy even more closely in to the plotline of that cartoon, there was even an oblique reference indicating that the head was designed to replace the head of the Robots in Disguise version of Fortress Maximus. I'm guessing the toy wasn't designed to actually do that, but if you remove all the spider-legs from it, it actually fits reasonably well.
Of course, if it's a Transformers toy, it has to transform, right? (Don't answer that) So Hasbro created a robot mode for the toy, which more or less looks Megatrony... or at least, it looks more or less like the Beast Era Megatron it was originally designed for. It really doesn't look at all like the Robots in Disguise Megatron that the toy bio says it is. But, oh well. This robot mode is rather unusual among Transformers in that its hands are not in fists (nor are the fingers articulated, which is what happens for most non-fist Transformers' hands). Even still, this toy is one of the better articulated toys of its era (an era when articulation was becoming more and more important). Other features of the toy include a flip-up missile launcher (and firing missile, of course) and wheels on the bottom of the head mode geared together so that when you push it, the spider-legs move up and down, simulating walking. This is a toy with lots of play value.
Perhaps it's not surprising that such a fun mold was recolored a couple of times. The version seen here is the original. The second version was called simply "Megabolt," who was apparently an entirely new character (although this character has never actually appeared in any official fiction). The third version finally saw the mold used as the character it was created to be. Sadly, this version was only available in Japan, and is now almost impossible to find.