While I'm quite sceptical of this, I do think the topic of Tamagotchi Prototypes is an interesting one.
Expanding upon what Penguin-keeper wrote above, test products were given out in the late 90s to see peoples' responses to Tamagotchis. It seems like a similar approach was taken when the Tamagotchi Connection released, as Bandai released
early adoption forms for people to apply with. It seems that this was more for review purposes than for preview purposes however, so it's likely that all the devices released through this scheme were standard US V1s (ROM version 4.0).
There's also this one image taken from a magazine back in the 90s that shows some seriously early prototypes of the P1:
Seems to be three different stages of development with the device on the bottom being a lot closer to the final product.
A prototype version of the Tama-Go was shown off in February 2010 at the New York Toy Fair. Presumably none of the prototype devices were stolen from the event, so I'm not sure we'll ever see these devices again, but they had at least one noticeable difference from the final version:
This screen says "CARTRIDGE" instead of "CHARACTER". The shell designs and faceplates were also vastly different from the final release - a shame, because the shell colours and a couple of the faceplates were really nice. A few character figures that were never released were also shown at the event.
Speaking of unreleased shell designs, I think pretty much every Connection version had at least one shell design that was shown to the public but never produced (I guess this isn't strictly about prototypes, but it's tangentially related).
These designs - both recolours of wave 1 designs - were originally intended to be released in the second wave of US V1 designs, but neither of them were.
Similarly, this graffiti design was intended to be released in one of the later waves of US V2 shells, but it was never seen again.
This design was also featured in the TV ad for the V2, along with another unreleased design:
There were also a number of designs for both the V4 and V5 which never got released, but since this post is getting quite long already I think I'll get back to talking about the software side of the prototypes.
I found this interesting status update earlier:
https://www.tamatalk.com/IB/profile/196422-gotchiguardian/?status=68920&type=status
Assuming it's real, I guess it confirms that there are prototypes out there in circulation.
Another prototype which may or may not exist somewhere is the Tamagotchi Music Star World Tour Edition - this version, which was basically going to be like a "Version 6.5", was revealed in early 2009 and cancelled later that year, with Bandai claiming the version never got produced. Perhaps somewhere in the world the design documentation for this version - or perhaps an early build - still exists.
Last but certainly not least, there does still exist some development and patent documentation out there detailing early versions of the game. I think there's probably a lot of documentation that's still unseen to the general public, but the few documents we have reveal some interesting things. I'll be detailing what I've found in these documents in my ROM versions thread soon.