"Squirrels" refers to the individual or group who originally dumped the data from the physical cartridge. Clean Dump:

Most modern Pokémon modifications (ROM hacks) require the Squirrels 1.0 base because the game's internal memory addresses differ between versions.

Pokémon Fire Red is a classic game that was originally released in 1996 for the Game Boy. It's part of the second generation of Pokémon games, known as Gold and Silver, but Fire Red is a remake of the first generation games, Red and Green (later released as Red and Blue outside of Japan). The game has a dedicated fan base, and its nostalgic value is high for many players.

Given the lack of official documentation, the most common interpretation among Reddit and GBAtemp users is that "1636" is the , and "Squirrels" is a corruption or user-added tag —perhaps from a YouTuber or streamer who used that name for their personalized ROM.