I think the Pikachu one comes from associating it with Pichu

https://pokemondb.net/pokedex/pichu

Its actually not, back in the 90s due to communication issues, poor management of merch and inconsistent official art avaiablity during the red/blue era. A lot of odd ball designs have existed for a number of pokemon.

Pikachu with a black tail tip, brown, a garident all exist. The early water colors being poorly maintained also caused issues with it. Not to mention third party manufactures basing it off pokemon card art or art from the actual artists that never actually was ment to be reference material.

I use to have a number of Pikachu toys as a kid. There were more designs then you can count. Fat Pikachu was highly inconsistent.

Now I have to dig through my art because I remember drawing that pikachu tip and I was only into pokemon for gen 1