As part of the Pokémon 20th Anniversary celebrations various Mythical 'mon are making their way into the current-gen games. There are monthly distributions to that effect, and the June 'mon should be dropping into the games right about now - it's the turn of Manaphy this time around.
This one originated in Pokémon Diamond & Pearl, though its "Egg first had to be obtained in the Nintendo DS game Pokémon Ranger", just to make things more complicated. It's a Water Type with the ability of Hydration, which is about as logical as one can expect.
Its other moves are Tail Glow, Bubble and Water Sport, and as normal with these particular distributions it's level 100. The distribution is via the Nintendo Network through the usual process for receiving and accessing online gifts in X, Y, Omega Ruby and Alpha Sapphire; if you're not sure of all the steps the official website has a handy visual guide.
This Manaphy is available from today (1st June) until 24th June, so don't forget to pick it up.
Also, if you're in Europe or Australia and don't yet have the Mythical Mew, code MEW2016 will unlock it for you.