Something not defined as a chicken would have to lay an egg that hatched something defined as a chicken at some point. Otherwise we couldn’t have chickens.
Yep. But at least with our current definition of what a "species" (roughly a group of organism that can interbreed and have fertile offspring), that's not possible.
If some not-chicken would lay an egg that hatches a chicken, that chicken would have nothing to breed with. It would basically be a genetic defect that makes it infertile.