From Destinypedia, the Destiny wiki
Very little is known of Starhorse, however it has been seen capable of summoning seemingly anything and anyone from across the paraverses to participate in Dares of Eternity, a game show of its own design. Along with this its seen capable of speaking directly into the mind of those it wishes to, however without Xûr, Agent of the Nine to translate its words, they come across as mere brays and neighs. Starhorse has been seen capable of manipulating ability-enhancing energy; from instantly recharging grenades and melee skills, to being able to supercharge Guardian's Super recharge time and mobility. Starhorse has also been seen capable of bestowing a perk known as Starhorse's Favor which allows the wielder to endlessly generate heavy ammo for themselves.
How Starhorse is capable of performing all this is unknown.
List of appearances