Hi Makiasan, Definitely "one". However, A) Have you ever ridden my horse? B) Yes, I've ridden it once.
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Mark answered:
A) Have you ever ridden a horse?
B) Yes. I've ridden [it/ one] once.
Regards told me;
Definitely "one".
Why make it more complicated than necessary?
Q: Have you ever ridden a horse (before)?
A: Yes, I have.
It's as simple as that.
Regards