It is mostly a title given to those like myself and my sister, who bear both horn and wing. My sister and myself are the only proper royalty in Equestria, though there is also Princess Mi Amore Cadenza, who rules the Crystal Empire up north. Twilight Sparkle was recently granted the title and blessing as well, and is currently tasked with maintaining order in Ponyville and its neighboring environs.

There are no others, to my knowledge.