A palette swap is a character with a similar body type and animation to another character. Palette swaps have superficial cosmetic differences and usually a few different attacks. Punch-Out!! for the NES and Super Punch-Out!! for the SNES make extensive use of palette swaps to conserve computer memory.
Punch-Out!! & Super Punch-Out!! (Arcade)
Punch-Out!! (NES) Palette Swaps
King Hippo has no palette swap.