Gorillaimo

Gorillaimo is an unreleased Pokémon that was just created as an example of the games' mechanics, showing the Pokémon battle system, its typing is currently unknown.

The only known information about this monster is just its name.

It was featured in a Game Freak book showing the origins of Pokémon, back in the day when the project was called Capsule Monsters and in that book we can see Buhi along with other unreleased and early designs such as Buhi, Papyo, Godzillante, Rhydon, Geodude, Gengar, Nidorino, Dragon4 and Kazeniga.

Appearance
Gorillaimo might be a medium sized to big ape monster, it is seen standing on its four legs but its safe to suppose it could've been able to walk or at least stand in its hind legs. The most unique characteristic that we can find on this design, is the baseball cap that it sports on its head. It also has an odd resemblance to Slaking.

As its name suggests, it is based on a gorilla.

Its face just shows its eyes, ears and mouth.

Fan speculation
Even if there is not enough information available on Gorillaimo, fans have some speculative theories, mainly on the design's origin rather than the actual Pokémon.

Fans say this monster could've been just a gimmick Pokémon based on another important Nintendo character, Donkey Kong, it of course lacks the tie that DK has, but its cap could be reminiscent of DK's little companion Diddy Kong.

Other fans think it could have been a reference to Ninten from Mother (EarthBound) video game developed by none other than Creatures Inc., formerly known as Ape Inc.

Based its looks, we can speculate on Gorillaimo's typing, it could've been a Normal type or a maybe Fighting type.