As a kid, it felt as if any transforming robot other than the Transformers was a copycat. I have no doubt that a number of them were. But the Transformers may not have been the first, so
What was the first depiction of a humanoid robot able to transform into an unsuspected object?
Answers should ideally address the differences between autonomous and (human) operated robots, and the differences between current objects and objects that are futuristic themselves, like spaceships.
I'm going to digress quite a bit from "robots turning into unsuspected objects" in this answer, because the creation of that theme was the culmination of several parallel trends in the anime industry and in the Japanese toy market.
The tricky part of this question is that all of the earliest transforming robots in childrens' entertainment were by no means "in disguise". They were openly fielded as combat vehicles, which then transformed into robots for a power-up. The first notable transforming robots in anime appeared in the mid to late 1970s. These series - Getter Robo, Raideen, Gaiking, Combattler, Voltes, and Daimos - all involved outlandish sci-fi vehicles which transformed or combined into giant robots. This was a natural evolution from the success of the giant robot anime series Mazinger Z and Great Mazinger, where the robots were piloted by a small flying vehicle that docked inside the robot's head, and the robots were granted the power of flight themselves via wing backpacks. Transformation just integrated the vehicle, flight unit, and robot into one.
As a result of this trend in super robots, the Japanese toy manufacturer Takara started the Diaclone toy line, one of the precursors to Transformers. Diaclone debuted in 1980, the same year that Denjiman, a Super Sentai series (the precursors to Power Rangers), first allowed the robot piloted by its costumed heroes to transform. Initially, Diaclone toys were much like the anime super robots that inspired them, bearing no resemblance to real-world vehicles. And much like those anime series, and Super Sentai series, Diaclone featured human pilots fighting monstrous aliens by pitting their giant robots against the aliens' forces.
By 1982, though, Takara had introduced the Diaclone "Car Robot" and "Jet Robo" series - toy robots that transformed into real-world cars and aircraft. These were still war machines piloted by humans, but they provided the toys that became many of the Autobots and Decepticons for the first two years of Transformers.
In 1983, Takara also added transforming real-world objects to their Microman toy line, a series based around two warring factions of 10 centimeter tall aliens, with the toys produced at 1:1 scale. The 1983 MicroChange series introduced toys that transformed into cassette players, guns, a microscope, and other items that will be familiar to the Transformers fan - these toys, as well, were incorporated into the early years of Transformers alongside Diaclone's Car Robots and Jet Robos.
There's a wrinkle introduced by Bandai's Machine Robo line, the forerunners of Gobots. These were released in 1982 as well, around the same time as the Diaclone Car Robots and Jet Robos. Machine Robo toys also transformed from robots into cars, aircraft, motorcycles and scooters, etc., but unlike Diaclone's piloted vehicles, these robots were sentient in their own right. Much like the story later developed for Transformers, Machine Robo depicted its robots as a species of alien robotic life forms engaged in a civil war. When Machine Robo was finally adapted into an anime series later in the eighties, though, the story took place on the robots' home planet, and did not involve the robots disguising themselves on Earth.
Another anime series also figures significantly into the chronology, though it did not contribute directly to Transformers or Gobots. In 1981, the TV series Gold Lightan (and accompanying Bandai toys) featured a titular giant robot, which transformed into an ordinary-sized cigarette lighter, as well as its robotic comrades, which transformed into a pocket monocular, a timer, and other small devices. The robots did use their alternate forms as disguise, passing as ordinary objects carried by the youthful hero until it was time to fight the villainous aliens.
So we see numerous competing and combining factors here. In the 1970s,the anime industry produced a lot of TV series about using transforming robots to fight aliens. Takara capitalized on this trend with their original toy lines, Microman and Diaclone. In 1981, Bandai brought out the Gold Lightan line, which may take credit for the first robots disguised as ordinary objects, but had little lasting impact. Takara and Bandai simultaneously pioneered the design of robots that transform into realistic vehicles in 1982. Bandai's Machine Robo took a concept of warring nonhuman aliens similar to that seen in Takara's Microman, but Machine Robo applied it to the transforming robots themselves. The 1983 MicroChange series of Microman toys first presented robots that were "in disguise" on Earth, though these robots were essentially tools of the Microman characters. And finally, in 1984, as Transformers and Gobots arrive in America, we see two competing TV cartoons establishing the American version of this story. The Transformers cartoon takes Diaclone and MicroChange designs and makes them into two factions of sentient alien robots hiding out on Earth, while the Gobots cartoon does more or less the same thing, except that the Gobots arrive on Earth openly, as I recall, and don't need to use their transformations to hide from humans.
The result of all this is that Transformers is probably the first work to present "robots in disguise" as you imagine it. But Transformers was a synthesis of the trends of the time in anime and robot toys. Before Transformers, MicroChange and Gold Lightan had popularized the idea of robots disguised as innocuous objects, and Diaclone and Machine Robo had more-or-less simultaneously debuted the concept of robots transforming into ordinary vehicles (though not necessarily for the purpose of disguise).
In the film Metropolis (1927), Maria the Robot transforms into an android. I'm not certain you could describe the female form she takes on as an "object" but she's certainly "in disguise"...
According to the Astro Boy wiki, Pook first appeared in “Crucifix Island”, which was published in 1958. He is a robot that can transform from a human form to different animals. Metropolis doesn’t seem to count based on the video, which does not show Maria changing into a non-humanoid form or having multiple forms.
