There will be a legendary/psuedolegendary based on the boss Plantera from the game Terraria, called Rosadion. This pokemon, a pure Grass type, will have four new moves, two designed for it but can be learned by others (Poison Seed, Thorn Ball), and two actually exclusive to it (Grappler, Fury of Thorns). All four of these moves will be Grass type.

It gets a signature ability, Jungle Wrath, which allows it to switch between two forms. Get ready, this is confusing as crap;

Jungle Wrath: when above half hp; gains the effects of Skill Link. 

When below half hp; it loses access to the moves Bullet Seed, Poison Seed, and Thorn Ball, and gains access to Fury of Thorns (Grappler, and all its other moves, can be used in both forms). Base attack is multiplied by 1.5x and base speed is doubled. If damaged, at the end of the turn, along with its regular move, it uses a move called Lethal Spore that cannot be learned naturally and only in this one instance. It basically works like Guillotine, but is Grass type.

Rosadion can be found in the Cavern Undergrowth, a rainforest-like cave, in southwest Campania.

A guardian legendary that is unnamed so far will play an important role in the story. It resembles a dragon/golem and is a Rock/Fairy type.

This legendary so far has an undecided location.

A mythical called Cosminox, based on a black hole, can also be found. It is a Dragon/Dark type and has a signature ability, Gravity Pull.

Gravity Pull: Prevents escape. Lowers the evasiveness of opponents by 2.

Cosminox's location will not be given.