Every Pokemon generation has its set of legendary Pokemon to discover such as Articuno, Zapdos, and Moltres in generation one and Groudon and Kyogre in generation three. Naturally that means that Pokemon Sun and Moon will have at least one set of legendaries and Nintendo has recently revealed them to be Solgaleo and Lunala.
Solgaleo is a Psychic/Steel type Pokemon with a brand new ability known as Full Metal Body which prevents Solgaleo's stats from being reduced due to an opponent's moves or ability. Lunala is a Psychic/Ghost type Pokemon that also has a new ability called Shadow Shield which causes Lunala to take less damage from an opponent's attack when Lunala has full HP.
If Pokemon Sun and Moon are like past games in which each has their own legendary, this pretty much swayed me towards getting Pokemon Sun as I like the Psychic/Steel type combo and I also like the idea of not having my stats reduced. Psychic/Ghost is a pretty good combo as well as Ghost types don't take damage from a lot of other types, but its ability just seems kind of meh to me.