Filling in the Pokédex in "Pokémon FireRed" requires collecting all the regular in-game Pokémon, all the legendary Pokémon and certain other Pokémon that can only be acquired by trading with other players. According to strategy site Bulbapedia, many Pokémon are only found in the LeafGreen version of the game and can only be obtained by trading with a player that owns that version.
According to strategy site Psypoke, there are ten legendary Pokémon to be collected at different levels of the game. Articuno is found in the bottom level of the Seafoam Islands. Zapdos is found by the exit of the Power Plant. Moltres is found at the top of Mt. Ember. Mewtwo is found in the Unknown Dungeon. Raikou, Entei and Suicune appear randomly on the world map after the player defeats the Elite Four, but the appearance of each one is based on the starting Pokémon chosen by the player. Lugia and Ho-Oh can be found by taking the right path and the left path on Island 8, respectively. The defensive version of Deoxys appears only in the LeafGreen version and must be traded with another player. Many other Pokémon appear in the LeafGreen version and not in the FireRed version, including Sandshrew, Vulpix, Bellsprout, Slowpoke, Staryu, Magby, Pinsir, Marill, Misdreavus, Sneasel, Remoraid, Octillery and Mantine. A player wishing to fill in the FireRed Pokédex must trade to acquire these Pokémon.