>>40953971
kabuto did perfect snake sage mode. when naruto was learning sage mode the toads explained that if you go too far you'll turn into a toad. kabuto took on too many snake like characteristics for it to be considered perfect or complete. hence mitsuki can go back to normal after using sage mode, but kabuto gets stuck with a tail and snake skin.

kabuto merged with the snake where mitsuki mastered the snakes chakra