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Fire Red is the remake of Japanese Red, while Leaf Green is the remake of Japanese Green.
Visual Differences
The cartridges, loading screens and end credits of the respective games are, obviously, different.
Version Exclusive Pokémon
These Pokémon can only be found in Fire Red:
- Ekans, Arbok
- Oddish, Gloom, Vileplume, Bellosom
- Psyduck, Golduck
- Growlithe, Arcanine
- Shellder, Cloyster
- Elekid, Electabuzz
- Scyther, Scizor
- Wooper, Quagsire
- Murkrow
- Qwilfish
- Delibird
- Skarmory
- Deoxys Attack Forme
These Pokémon can only be found in Leaf Green:
- Sandshrew, Sandslash
- Vulpix, Ninetales
- Bellsprout, Weepinbell, Victreebel
- Slowpoke, Slowbro, Slowking
- Staryu, Starmie
- Magby, Magmar
- Pinsir
- Azurill, Marill, Azumarill
- Misdreavus
- Sneasel
- Remoraid, Octillery
- Mantine
- Deoxys Defense Forme
In-game Trade
The Pokémon asked for in the in-game trade at Route 18 in FR is Golduck, while it is Slowbro in LG.
Sources: This, this, this, this, this and this video.
Hope this helps!