So, I've seen the snail planet in a previous playthrough, but I haven't gotten near it in this one yet.
A child of two regular Earth bunbons has a snail shell anyway. "Mystery Shell" also has a child who (while unrelated to the deer bunbon) got antlers.
I'm not complaining -- I actually think this is pretty cool -- but I'm curious whether this is technically supposed to happen or not.
EDIT: I've found something which I am complaining about! I'll pause the game and come back to one less bunbon, and the logs will say that that bunbon blasted off when I wasn't looking. Which sucks. I haven't found out yet if these bugged-out spacefarers are waiting for me on the end screen or not.