This one is fairly simple to do. Find a horse (these spawn in plains biomes, which are flat grassy areas), and walk up to it. Make sure there's nothing equipped in your hand, so you only see your fist on screen. Then, press L2 to mount the horse. After a moment, it should throw you back off. Keep mounting the horse until eventually, love hearts will pop out of the horse. This means it is "tamed", and then the trophy will pop.