Moltres in depth
A Spanish numeral at the end of the name
The English names of the three birds end in a number in Spanish: Articuno ends in uno, Zapdos in dos and this one in tres. The first half is molten. In Japanese there's no wordplay at all — ファイヤー is read “Faiyā” and is the English word fire written in katakana, with no double meaning or hidden joke. French went in another direction: Sulfura combines sulphur with Ra, the Egyptian sun god.
The design looks to the phoenix, and the connection is written into the creature's own description: according to the game's internal legend, it's the embers it sheds with every flap that gave rise to the myth of the burning bird. The rest of the body is an ordinary large bird — burning crest, reddish legs, yellow plumage — with the fire placed only where it's visible.
What changed between generations
In Generation I there were no abilities, so for two games it was just a pile of stats. Pressure arrived in Generation III and Flame Body was added later as a hidden ability, which is the one that matters in a battle where the opponent attacks by contact.
The big change came earlier. In Generation I it had a single Special stat; when that was split in two, the offensive half stayed at 125 and the defensive half fell to 85. That 85 is now the weakest part of its sheet. Then came the second blow with entry hazards: ever since Stealth Rock existed, any Fire and Flying type pays half its life to enter, and a legendary that needs to rotate out to use Roost is exactly the profile that suffers most from it.
The order in which it learns things
Its level-up curve is slow and disorderly. It starts with Gust and Leer, takes until 5 to get Ember and until 15 to get Wing Attack. Its first decent Fire-type hit is Incinerate at 30, with 60 power. Air Slash at 35, Roost at 40, Heat Wave at 45. Overheat doesn't appear until 65, and Fly, with its 140 physical power, not until 70 — by which point it isn't needed anymore.
The oddity in the list is at level 25: Ancient Power, Rock type, 60 power. It's its only answer to other Fire and Flying types, and against a Charizard or a Talonflame it settles an exchange that would otherwise be awkward. Sunny Day at 50 fits the rest: in sun, Heat Wave and Overheat get stronger, and Solar Beam comes in through a TM without a charge turn.
The black bird of Galar
The second form is “moltres-galar”, added in The Crown Tundra, the expansion for Sword and Shield. It swaps fire for darkness: it's Dark and Flying type, with a black body, scarlet details and blue eyes, and it comes with its own ability and move that it doesn't share with the Kanto version. It shares the Pokédex number and general silhouette, little else. Anyone looking for the fire bird won't find it there.