More backstory/lore to Butterfly Wishes: Butterfly Wishes was created using a machine that Lovestruck used called the “OTP child machine”. It would generate a child based on any shipping between my little pony characters. In this case, Trixie and Fluttershy. So technically, Butterfly Wishes was just a lab experiment. Now how did she become an alicorn? Well, there’s an easy explanation for that. Butterfly Wishes was “born” an alicorn because simply… MIX OF D N A
Since Fluttershy is a Pegasus, and Trixie is a unicorn, fusing those traits will create an alicorn.