I've got three current fursonas with different stories as to how I picked their species.

Tofu (Zenko Kitsune): I actually had a lot of trouble narrowing this one down, and the final decision literally came down to being on a discord server about foxes and what I could do with DazStudio. The character that became Tofu could have been a Kobold, a Land Form Shaymin, a Sphinx (as in the Greek/Egyptian mythical creature, not a Bingus) or even a "scrap yard special" protogen with cybernetics made from recycled and/or repurposed parts, but eventually I decided on a Zenko Kitsune with five tails, who's tails represent her age in centuries.

Sloan (Serval): If you can't tell, Sloan's my main. Tofu is very much not like me at all but rather more of a way to self-explore, so I decided I wanted to make a fursona that could act as my mascot. I thought about what traits are typically associated with various fursona species and settled on "lesser" wildcat, and then went with a serval because I'm tall and so are servals.

Amara (Land Form Shaymin): I liked the Land Form Shaymin concept I had when trying to decide on Tofu's species, so I decided to use it as a separate character/'sona.