Kurusu, the unreleased Gen II water starter.
Kurusu is a Water type Pokémon, it is currently unknown if it had a secondary type, but it's unlikely.
It has no known evolutions, but it is most likely the basic stage of a 3-staged Pokémon evolution family.
Along with Happa and Honoguma, Kurusu is one of the original starter Pokémon available only on the Gold and Silver Demo.
The moves that Kurusu knew were Growl, Water Gun and Tackle, and apparently its Pokédex number was 212.
The original art that features it is most likely a fan version, heavily based on the Gold and Silver Demo sprite.
Kurusu is still an unreleased Pokémon, originally to be released in Gen II.
Appearance
Kurusu is a small white or light blue small seal-like creature, it has four flippers, a normal-sized tail, and two ear-like appendages on its head.
It is unknown if it's based on a mammal, a reptile or a slug type animal.
Its face does not show any other traits rather than its eyes, although in some fanart, a mouth is seen.
Fan speculation
Fans have said Kurusu could have been an early design of some official Pokémon, like Seel, Spheal, or Shellos.
Some fans have speculated this could've been a planned Lapras pre-evolution, but this seems unlikely due to the fact that Kurusu was meant to be a starter Pokémon.
A known fan Pokémon that resembles Kurusu was featured on the Pocket Monsters Manga, it was called Marin and it was only featured once.
Notes
- It should also be noted that the Pokemon looks a lot like a fan Pokemon named Marin, who appeared in the Magical Pokemon Journey Manga, after a "Draw Pikachu's Friend" competition.