Rainbow Ponies were My Little Pony releases with rainbow-colored hair.
The original Rainbow Ponies had manes with stripes of dark pink, yellow, green and blue hair, while their tails had these same colors mixed together. They had glittery symbols, and it's common for the glitter to wear off over time.
The Rainbow Ponies first appeared in Year 2 of the G1 line, with a second set released in Year 3. Rainbow Curl Ponies, with tight curls instead of straight hair, were released in Year 8. These ponies had different colors in their hair than the original sets.