Laketoise is an unreleased Pokémon and its typing is currently unknown, but based on its looks, it's safe to assure it was a Water type (at least as a second typing).
It has no official or confirmed name, "Laketoise" was just a fan name given (by cSc-A7X) to this unreleased Pokémon just to refer to this particular monster the easiest and most recognizable way possible.
It was featured on the Cover Art Gallary 1997 Vol. 14 with the original art being completely official and made by Ken Sugimori.
This document shows Laketoise among Peritop,Tyranitar, an unknown Pokémon male trainer and an unknown female trainer.
Laketoise is still an unreleased Pokémon, and was to allegedly be released in Gen II.
Laketoise is a medium to big sized turtle which most prominent color is light blue. It has four legs with darker blue-green details and three sharp claws on each of its feet. It has a big dry grass green colored shell with some details being cream colored, and darker green spots on the shell.
It is based on a land turtle or tortoise.
Its face and neck are partially dark blue-green, and it features eyes, ridged mouth and nostrills.
Some fans think it could have been an early design of other turtle-based Pokémon like Torkoal or Turtwig. Others have really odd or farfetched theories that it could have been an early design of Lugia or Shuckle.
Theories and interest for Laketoise was revived when Gen V Pokémon started emerging/being released, the reason behind this was the reveal of Tirtouga, a turtle-based Water/Rock Fossil Pokémon.
And while some fans believe that there is no reason to relate or even "confirm" that in fact Tirtouga was a redesigned version of Laketoise, finally released by official means, just because they were based on turtles but one of them was a land turtle and the other one a sea turtle, it does not seem impossible to believe.
As you may know, Gen II was the only Generation to NOT have or release any Fossil Pokémon, which might lead us to believe, if Laketoise was confirmed to be released in Gen II alongside Tyranitar (for example), couldn't this monster be the Fossil Pokémon we did not got, and it was redesigned and finally released 13 years after its first appearance?
And looking at the design, even if it has some absolutely different shapes and features, the face seems to be very reminiscent of the turtle we never got.