science

I'm no biologist. It's very possible it's an incomplete definition, and I don't claim it to be a perfect one.

I guess if we apply my definition to mules, each mule would be a different species lol.

The horses one is a non-issue though. It doesn't matter that they can create offspring of different species. Since 2 horses can potentially create a horse, then the horses are of the same species.

And yes, my definition works only for sexual reproduction, since as seen by this article, asexual reproduction can get very complicated.

I wouldn't say it's outdated and mainly for children. Just like Newtonian physics are very useful if we use it correctly. Having simple models that work in the situations we encounter most is useful even for adults.