This one is going to be a little more "out there" because I'm looking for something I read on the internet; I don't know if it's ever been published in any physical form.
I don't remember the plot of the story at all, but it's about a human who is one of the last of the humans. Humans using some tech (might have been nanotech) have tried to cure all ailments by eliminating mutations. The story specifically mentioned that 'we included our symbiotic bacteria', but after a while, humans are dying because the environment changed and we (humans) are no longer to adapt. As a result, we started uplifting other species, including horses, I'm pretty sure elephants, and probably dolphins and chimpanzees though I don't recall them specifically. Several of these had... issues. The horses were the most prominent... and they sang. Initially, this is a strange and confusing habit, but whatever goes on in the story, it ends with the POV character saying a line very much like "now I know why the horses sing".
Does anyone remember this and know the author/title? Is there a copy online somewhere?