Muneo Saito is best known to Pokemon fans for the work he did on Pokemon Gold and Silver, as well as the third generation games. Back in 2014, Saito published some creature designs that he did for an unspecified video game. Twitter user Dr. Lava, who often posts images and information about abandoned Pokemon designs, posted the art, which has fueled speculation that it was possibly meant to be a long lost Pokemon.
The art shows a creature that has four stages in its evolution. The creature starts life as a popcorn kernel or seed, but then becomes a cornstalk-like thing. From there, it evolves into a monster with what appears to literally be corn on its head, until it finally reaches its final form - a bearded creature with popcorn pouring out the top of its skull.
Since Saito didn't clarify whether this design was meant for the Pokemon series or something else, we can't really say for sure that this weird popcorn Pokemon is actually a long lost Pokemon. The fact that it evolves certainly seems to work in its favor, but it having four forms makes it a little more suspect. After all, most Pokemon that can evolve usually only have two or three forms, so it this popcorn creature could very well have been meant for another game.
If this popcorn creature was meant for the Pokemon series, fans shouldn't hold out hope that it may eventually be added as part of a future game. The Pokemon Company doesn't really seem to revisit abandoned Pokemon designs all that much, as is evidenced by the unused Pokemon found in the Pokemon Gold and Silver Space World demo.
However, that doesn't mean unused Pokemon designs won't necessarily inspire future Pokemon. For example, Farfetch'd had an evolution that was discovered in the Pokemon Gold and Silver Space World demo, with some speculating that the unused Farfetch'd evolution could be used in Pokemon Sword and Shield. An accurate leaker has said that Farfetch'd has an evolution called Sirfetch'd in Sword and Shield, and while we have no idea what Sirfetch'd looks like at this point, maybe it will resemble the unused Farfetch'd evolution.
It will be interesting to see if Nintendo ever revisits the concept in the future.