Connect Wall: "Who's that Pokémon?"

Question

^{An entry in Fortnightly Topic Challenge #42: Wordless Connecting Walls}

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Ash, on his expedition to Catch Em' All™, came across a strange new Pokémon! He pulled out his trusty Pokédex to identify this mystery Pokémon, but rather than directly revealing this it, it instead gave Ash sixteen Pokémons it had already encountered! The Pokédex said the following:

There are sixteen Pokémons underneath who fit nicely into four groups of four. The Pokémon you seek will fit into all four categories.

Can you help Ash identify the Pokémon?

Quick clarifications:

• The type that Arceus is in is somewhat insignificant. For those who are curious, it is currently in normal type and not holding any plates.
• Both forms of Aegislash were given to avoid confusion between sword/shield forms. I put both so that people wouldn't be accidentally led aside thinking that the specific form it was in was significant.
• I've updated the connect-wall board, getting rid of the Gen. VIII Pokemons. Also, because this is causing a lot of confusion, it is NOT about changing forms.

Hint 1:

Three of the four category themes would be helpful to know in a Pokémon battle.

Answer 1

Here are the four groups I found:

Group 1: The names of the Pokemon contain their types.

Dragonair
Rockruff
Steelix
Regice

Group 2: This pokemon has the highest base speed stat for their type

Deoxys (Speed Forme): Psychic, 180
Ninjask: Bug/Flying, 160
Electrode: Electric, 150
(Mega) Aerodactyl: Rock, 150

Note: I added the different forms for Deoxys and Aerodactyl because that gave a higher base speed stat, but it must be noted their normal forms still have the highest base speed stats for their respective types. Normal Deoxys has a speed stat of 150 and normal Aeordactyl has a speed stat of 130. This will be important later.

Group 3: This pokemon is part of a duo.

Cresselia. (Lunar duo with Darkrai)
Latios. (Eon duo with Latias)
Ho-oh. (Duo with Lugia)
Phione. (Sea guardian duo with Manaphy)

Group 4: This pokemon has its own signature ability, unique to itself. According to OP, this group also has its own signature move.

Xerneas: Fairy Aura (signature ability), Geomancy (signature move)
Victini: Victory Star (signature ability), Searing Shot and V-create (signature moves)
Arceus: Multitype (signature ability), Judgement (signature move)
Aegislash: Stance Change (signature ability), King's Shield (signature move)

Hence, based on the four groups, the mystery Pokemon seems to be Darkrai.

1) It has Dark in its name.
2) It is the fastest amongst Dark-types, having a base speed stat of 125. This is only true if we exclude Ash-Greninja, which has a base speed stat of 132. The original Greninja has a base speed of 122, so if we only consider original forms, Darkrai is the fastest (sharing it with Weavile).
3) It is a duo with Cresselia, as mentioned.
4) Darkrai has its own signature ability, "Bad Dreams". Its signature move is Dark Void.

Answer 2

EDIT: After being adviced by more experienced users, I now change this to a community wiki answer. Feel free to use the information here!

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Not sure if this counts as spoiler.

For those who are not familar with newer generations (like me), here are the pokemons in the picture.

Finding them already took me some time, so I hope this could help others.

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#716 Xerneas

#386 Deoxys

#291 Ninjask

#493 Arceus

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#148 Dragonair

#488 Cresselia

#101 Electrode

#489 Phione

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#494 Victini

#208 Steelix

#378 Regice

#381 Latios

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#142 Aerodactyl

#250 Ho-Oh

#744 Rockruff

#681 Aegislash