This page of the Assassin's Creed: Valhalla trophy guide describes how to unlock the Equine Attack trophy. This achievement is awarded for killing an opponent by jumping on him from a horse.
Just jumping off a horse can prove quite troublesome. When traveling on horseback, press the Circle / B. This will make Eivor take the squatted position ready to jump - it is shown in the picture above.
It is best to unlock the trophy in one of the cities. You may encounter opponents there, but it is not that the whole city is a forbidden location. Carefully approach your opponent from behind on the horse. After you get close enough to the enemy, you should be informed about a possible silent blade assassination - you should, of course, use it by pressing R1 / RB.
If you correctly landed on your opponent and killed him with a silent blade, you will unlock the trophy.