Indeed. I didn't say offspring. As natj points out...... I said FERTILE offspring. Fertile is absolutely the key word overlooked.
"Species
A species is often defined as a group of organisms that can reproduce naturally with one another and create fertile offspring. However, the classification of a species can be difficult—even riddled with controversy."
https://education.nationalgeographic.org/resource/species/
It's been an acceptable basic definition for a long time. I alluded to the difficulty and controversy in my post.
Zebra horse hybrids in either direction are also sterile.
Ligers and tigons are other examples. They rarely produce fertile offspring. Ie: they are sterile. They are compatible enough on a chromosomal basis to produce viable offspring. Even displaying significant hybrid vigour. But that offspring cannot reproduce. So like a mule they are doomed to never be biologically classified as a species.
It's different story with fish. Different species can interbreed and have viable offspring. Creating something new. But getting those offspring to breed true to type is another matter. But it can happen. It happens in nature, and in aquariums. Which is perhaps why there are so many locality variants and subtypes in places like the African rift lakes and coral reefs. In aquariums we have things like flowerhorns which are controversial.
Like fish, I imagine there is a bit of this that can happen in reptiles and birds. Especially when geographical isolation occurs. It's seems a trend that the higher the order of animal, the more compatible they have to be to create fertile offspring. But I'm not a zoologist, amd nature has a way of breaking the rules.
There is so much we don't understand.
On peafowl and chickens:
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