is it ok if... a trans mare say

TransMare: "Oh, when I was a little colt, everypony keeped mistacking me for a filly"

edit:

TransMare: "Oh, when I was a little colt, everypony kept mistaking me for a filly."

is that ok ?
or should I used something else ?

It sounds fine to me. Besides the grammar issues. It should be "Oh, when I was a little colt, everypony kept mistaking me for a filly."

Like Sunshyne pointed out, the grammar needed to be touched up. Without more context, it's hard to judge what's being said. Personally, I'd probably shy away from anything that might infer that them being seen as a filly was a mistake if the pony is transfem. Reaffirm their confidence in their identity.
For example, "When I was young, my parents only ever saw a colt, whereas strangers often saw me for the filly I really was."

7957488
opps sorry

7957502
Actual... the funny thing is that the TansMare is adopted, and her mom thought... that she was... fiscally a filly
turns out that... the mom wasn't that off

tbh at least in my perspective, I'd be more likely to say filly than colt if I was in her place. Mind likes to do the gender affirming things.

7958004
I understand

but it was to make it clear that she's talking about before she came out
and started... TH !?

do you know a different way to say it ?