Well, golly gee willickers! Pony has learned how to open a gate!
The past few weeks I've thought maybe I'm losing my mind as I would come out to the barn after having let all the horses into the sacrifice area for breakfast and have subsequently found them back in Mac's stall/paddock with the back gate open! At first I thought I maybe forgot to latch the gate. Then it happened when Colin was on breakfast duty. And then again and again and again. I wanted to see who was doing it before I went to chain the gates closed, and this morning I got a chance to see it.
Turns out Pony has learned how to open the gate! I don't know what/when/why/how she learned, but she did. Funny thing is that she hasn't done this to let herself OUT of her paddock at night, but to let herself IN to Mac's paddock after breakfast. And everyone follows her!