The "1636" prefix is a scene release number from the early GBA era, while "Squirrels" identifies the person or group who originally ripped the game data from the physical cartridge. The Squirrels dump is highly prized for two reasons:
A rarely-discussed ROM hack from 2017 called “Pokémon Squirrel Fire Red v1.2” exists on unofficial forums like Whack-a-Hack and PokeCommunity. Features: 1636 Pokemon Fire Red - U-- Squirrels
Post-Game: After beating the Elite Four and catching 60+ Pokémon, talk to Professor Oak to get the National Dex. This unlocks the Sevii Islands quest to find the Ruby and Sapphire gems. The "1636" prefix is a scene release number
The "1636" prefix refers to its release number in the global database of Game Boy Advance ROMs. The name "Squirrels" comes from the group or individual who originally dumped (digitized) the cartridge. Enhanced graphics over the original Red/Blue
Pokémon Odyssey: A complete overhaul based on the Squirrels base. Legal and Ethical Context
1636 - Pokemon Fire Red (U)(Squirrels) is the specific file name for a popular ROM dump of Pokémon Fire Red Version 1.0. It is widely recognized in the fan community as the gold standard "clean" base for applying ROM hacks and patches.
Pokémon Unbound: A completely new region and story based on the FireRed engine.