In every iteration of the Pokémon games, with exception to the first, each dual-version features legendary Pokémon on the box art. Pokémon Sun and Moon keeps that tradition alive with two brand new legendary Pokémon hailing from the Alola Region. We currently have not learned their names, but they were shown off in the starter Pokémon reveal trailer.
The cover Pokémon for Sun version is a regal lion. Its head has a galactic glow with a symbol appearing in the middle. Its mane is large enough to look like the outline of a sun.
The cover Pokémon for Moon version resembles a large bat. With its wings spread it forms a circle around its body to unleash powerful attacks. Actually, it resembles a crescent moon and full moon depending on how far the wings are stretched.
The outlines and markings of their bodies resemble the logos for the two games as well. It seems they were revealed this whole time albeit in less detail.