What even is a dragon? We just don't know.

In terms of franchise canon, a dragon is or seems to be any Pokémon that is Dragon type, part of the Dragon egg group, or carries “Dragon” in its name and description. Technically then, Mareep is or can be a dragon since both it can (Mega) evolve into a Dragon type, and its evolved morph carries “dragon” in its name (“Denryū” –> Ampharos).

In the Suocéverse the term is overloaded to refer, among Pokémon themselves, to the various dragon races / dragon species and the cultures they form. What counts as a dragon follows basically the same rules as in franchise canon, but each draconic culture has some different internal rules over whether eg.: Charizard are dragons or not.

In general, all major draconic societies share the same tale of cosmological events:

Arceus created the material universe, and gave form to raw power thus creating the Creation Trio who are henceforth Dragons, as well as “other beings” such as the Lake Trio. The Lake Trio were set out to populate the material world with living creatures, and a special cast of those ended up attuned to the Dragon energy and thus became the Dragon species. There were also a few particular cases where the Creation Trio themselves created dragons that shared their universal powers.

In a very general sense, only the members of the Creation and Lake trios can confirm or deny any of those tales. Suocéverse canon has it that the Lake Trio themselves consider “draconic” any Pokémon who is part of the Dragon Egg Group, and that the Creation Trio did create on their own at least one Pokémon: the Drao.

As of G9 Suocéverse, the mortal species that are considered by Dragons as “indisputably Dragon” or “pure Dragons” are the ones that are in the Dragon Egg Group and have the Dragon type in most or all their stages (not necessarily morphs) starting at base: “start as dragon, and never lose that capacity”. As per G9, the following species match this criteria:

Dratini, Bagon, Gible, Axew, Druddigon, Deino, Tyrunt, Goomy, Noibat, Turtonator, Drampa, Jangmo-o, Applin, Duraludon, Dreepy

Other species are considered dragons or not depending mostly on whether they form part of the Dragon Egg Group and if they obtain or lose the Dragon Type.

At the lowest end of the dragonhood spectrum is what pure dragons call “bastard dragons”: creatures who have no access to the Dragon type, who try to “pass” as dragons in appearance (eg.: Treecko, Helioptile) or belligerence (eg.: Charizard having a Dragon type Mega), and whose only “real” claim to dragonhood is that of bedding other dragons (ie.: being breeding compatible with Pure Dragons):

• Charmander
• Magikarp
• Treecko
• Seviper
• Scraggy
• Helioptile
• Silicobra

Feebas and Salandit are considered special cases or exceptions depending on the era, as they tend to show up with suspicious frequency among the ranks of select draconic ruling classes: such as Vedhismegral having been a member of the Seven Star-Claws.

See: Dragon Languages.

Dragons in the Suocéverse use a number of languages. Primarily there's Monese, but also they use a number of out-of-universe conlangs, partially adapted to reflect a Pokémon culture.

• Dragons of the Western Wings, such as the dragons in Dragonspiral Tower, as well as Reshiram, Zekrom and Rayquaza, use some elements of Dragonspeak (from FF XIV).
• Dragons of the Southern Wings, such as the dragons migrating as part of the Ísparus Draconic Claim as well as the Latii, use some elements of Thu'um (from Skyrim).
• Volyrina's dragon clan in the PMD Maniline world, and Zygarde in mainline, use a form of Welsh language as “Draconic”.
• etc.

As with other language-related story interface elements, the variety of languages is abstracted into English (or another publication language) most of the time.