When an Aura enters without being cast, if the effect that puts it onto the battlefield doesn't specify what it will enchant, you can attach it to any object or player that fits the requirements of its enchant ability. There are a few exceptions. You couldn't attach Pacifism to a creature with protection from white, for example. Attaching an Aura this way doesn't target the object you're attaching it to.

This is a trick I used in Brawl once to gain control of an opponent's Niv-Mizzet, Guildpact with some Mind Control enchantment (I forgot which). It rarely comes up, but it's one of those tools you can use to give you an edge in niche scenarios.