Well, after this excellent exercise of deduction Dan has made it’s clear that there is no need to labor the point, so I can only say that I agree with him. My choice is also “puggle”. Thank you, Dan. 😇

I am a US native speaker and I don't know the answer! I am going to explain my reasons and then enter my guess. I can eliminate "Joey" because I know that's a baby kangaroo. Birds that come from eggs can be "hatchlings" and echidnas lay eggs to that's possible, but it's sort of a technical word, like calling a human baby a "neonate." "Kit" can be a word for many baby animals, but I THINK "kitten" is only used with the cat family. "Puggle," I have no idea. Since echidnas are pretty rare and it is mostly likely only an indigenous person would know them well enough to need a specialized name for a baby, I guess I'll go with "puggle." Let's find out.

@dansmith koala, wombat and kangaroo babies are all joeys! :-)