Ask the guy who is pretty good at SF Story Identification

[Frodo]

@teelabrown

I don’t remember much, but some alien race had captured humans long ago and bred them to be service animals. The human breeds were as unalike from each other as different dog breeds, and they were used as pack animals, riding animals, etc.

That sounds like Vance’s “The Dragon Masters”

However this

@teelabrown

And some form of alien religion featured as well, as complex and dogma-ridden as some Earth religions.

Does not.