expression, godan verb (-ru)
get on a horse; mount a horse
Literal action of climbing onto a horse. The phrase is a straightforward combination of 馬 (horse) + に (particle) + 乗る (to ride/get on).
初めて馬に乗ったときは、とても緊張した。
I was very nervous the first time I got on a horse.
彼は馬に乗るのが上手だ。
He is good at riding horses.