Magic shapeshifting is cool too, but there’s something about a mechanical transformation with parts moving that’s so much more appealing to me, so what I’ve always really liked have been robots, mecha, and other machines that change into other forms.
I like transforming mecha enough to make them prominent in my own writing. In Project Quintessence, the mecha called Variable Guardians transform between humanoid and animal forms, because I also love animal mecha and animal motifs in general.
So where did the idea of transforming robots really come from? Well, it actually developed gradually over time; transformation itself transformed.
Astro Boy actually had some combining robots, I’m pretty sure before anything else, but with a long enough time gap that I’m not sure it would be accurate to say it’s where later examples got the idea from.
Mazinger Z’s contribution to this is a borderline example. Koji entered the super robot by flying a small plane that docked with its head, so that he was literally the robot’s brain. Several other robots in Mazinger Z and Great Mazinger had vehicles that served similar purposes. Of course, the main bodies of the robots didn’t change.
Getter Robo was the first vehicle/robot combiner, and the first robot with multiple combined forms. The three jets could combine in three different configurations; Getter-1 for air (and in practice, the main form overall), Getter-2 for land, and Getter-3 for the sea.
I’m pretty sure that the first robot to transform without combining or splitting was Raideen. It was also the first transforming toy (due to Getter Robo’s transformations, you know, defying the laws of physics).
So the idea of transforming robots goes back to the 70s, but it really took off in the 80’s. Diaclone was a Japanese toyline where basically everything transformed.
Then the highly popular anime series Macross had its mecha transform into planes that looked a lot like F-14s. That began a sudden surge in toys (including a change in direction for Diaclone) and anime with mecha and robots that transformed into recognizable vehicles or objects.
Then, almost at the same time, Tonka’s GoBots and Hasbro’s Transformers brought a lot of these Japanese toys here with new stories and cartoons (GoBots toys were mostly from Machine Robo; Transformers used a lot from Diaclone and Microman as well as few from several other lines, even one from Macross), spreading the popularity of transforming machines spread to the rest of the world too, with countless less popular imitators. Not even just robots, either; there was also Kenner’s M.A.S.K. (Mobile Armored Strike Kommand…yes, they already used Xtreme Kool Letterz in the 80s), which was about ordinary vehicles that transformed into not-ordinary vehicles, like a car to a flying missile-launching car.
We also got Voltron (made by, somewhat appropriately, combining two anime series about combining super robots) and Robotech (made by combining Macross and two other series with transforming mecha) on TV in the west around that time as well, so basically, transforming/combining was everywhere in the 80’s.