The case
Pokémon hit the U.S. in 1998, and younger Gen X either played it or bought it for someone. Pikachu's black-tipped ears are one of the most-drawn details in kid art of the era, and a lot of those drawings put the black on the tail too.
Official art has never had a black tail tip. The ears carry the black; the tail carries a patch of brown at the base and lightning-bolt yellow the rest of the way.
Why our brains do this
The design has black tips somewhere, and memory generalizes the pattern from the ears to the other pointy part.
Sources: Wikipedia — Pikachu · Wikipedia — False memory: the Mandela effect
