Have alien rabbits been mentioned before this chapter
No, but there is some thing about rabbits and the moon in Japanese myths, so probably is a reference to that, like Goku in dragon ball classic sending a Villain to the moon because he was a rabbit, and rabbits belong in the moon.
there is something about rabbits making mochi in the moon or something, but i don't know much about it.
I really wasn't expecting to see it come up here either (not complaining), but It's a very well established myth/folklore thing that exists in multiple cultures: (and as mentioned, has become a trope (tvtropes link warning) adopted in anime/manga)
Moon Rabbit (Wikipedia)
- The Moon rabbit or Moon hare is a mythical figure in both East Asian and indigenous American folklore, based on interpretations that identify the dark markings on the near side of the Moon as a rabbit or hare. In East Asian mythology, the rabbit is seen as pounding with a mortar and pestle, but the contents of the mortar differ among Chinese, Japanese, Korean, and Vietnamese folklore.
- Moon folklore from certain Amerindian cultures of North America also has rabbit themes and characters.