Mushussu is a demension-traveling documenter, and owner of his own personal, multidimensional library. He is also the author of his own beastiary, categorizing numerous Mystical Creatures from various worlds.
Physical Appearance
Mushussu is a mish-mash of various animals much like Discord: he has the front legs and tail of a lion, the neck and head of a snake, the horns of both a ram and a Unicorn, and the back talons of a sparrow.
History
As he was writing up another part to his Bestiary series, he stopped over in an Alternate Timeline, in which Earth and Equus had met far earlier than the current timeline, and as such categorized various creatures such as the Thunderbird and Grootslang. He left this particular study within Beach City's Buddwick Public Library.
Season 5
After finding that his book had been taken, he went into Ponyville, and came across the Golden Oak Library to get it back. He met with Twilight Sparkle during this, and revealed that it was not just his book, but that it was him who wrote it as well. He offered her the option to visit his library if she wished, and gave her the directions to the library, which was currently off in Hollow Shades.
He was later found inside his library, toppling over a pile of books just before he met the rest of the Mane Six. He guided them to his personal bestiary section and left them to their studies. In a conversation with Rarity, he then revealed that he was from another dimension entirely, and that while he visits many dimensions and worlds, he barely had time to stay in his own. When pointed out of the picture of Quetzalcoatl, he explained that feathered serpents in his world were like Alicorns in Equestria, and The Diamond Authority to Homeworld. He also jotted the appearances of Mystical Creatures of Earth to the act of "Dimensional Rules", as he found a timeline where this already happened, and that their current timeline's altering to fit that same one. He allowed the group their own copy before they left.
Abilities
Dimensional travel
Mushussu is seemingly capable of traveling through different timelines and dimensions, as many of his bestiaries describe various creatures from different worlds. It's not revealed how he could do this, however.