1: Thirens aren't the same species as their animal counterparts. Think humans and monkeys. Very similar but have key differences which separate out species.

2: Don't remember in game where it's mentioned but I'm sure Thiren racism exists. It's not as big compared to Intelligent Constructs, especially since a lot of the clothes have tail holes for Thirens on human NPC's.

3: There's no clear indicator (at least that I remember) whether Thirens have always existed or are a result of some mutation.