The behavior of a herd stallion changes often and with a moment's notice. One minute,…
“I know what you are thinking. You are wondering, ‘How is it that he is so strong?’. The man smiled and tapped his finger against his temple. The reason I am so strong is because I think I am.” –Author unremembered
(Remind me if you know what book that quote came from).
In this case, the He would be She. The yearling is a filly.
An almost-yearling holds her own between a bossy two-year-old and a domineering older herd-mate.
Attitude and prudence is always a balancing act.