Favorite Background Ponies: Derpy Hooves (If he still is considered one), Icy Baronstar (Obviously), and Seafoam (Colgate is overrated :3) whats yours?
If she can be counted as a background pony, (i'm not sure because she is talking and interacting with th main characters of some episodes) then it would be Cheerilee. If not, i have to say Lyra/Heartstrings.
Just a FYI: if ponies have heads like the mane 6 do, then they're female. If they have heads like Big Macintosh, as well as unshorn fetlocks, also like Big Mac, they're male. That's how you differ a female pony from a male one.
And yes, mistakes are perfectly fine! Everypony makes them.