Could you provide more context? For example:

: It is considered the most stable and "clean" version of the game.

You wrote "Squirrels" in your prompt, but the image of is actually a funny coincidence. In the Pokémon games, there aren't many squirrel-like Pokémon (until Squawkabilly or Pachirisu in later gens). However, the 1636 ROM is often used to create randomized runs. It is a very common joke in the community to randomize encounters and end up fighting "Fire Red Squirrels" (fire-type rodent Pokémon) because of how the randomizer algorithms work.

What's the difference between different roms? : r/PokemonROMhacks

If you’ve spent any time in the Pokémon ROM hacking community, you’ve likely seen the phrase pop up repeatedly. For beginners, it sounds like a weirdly specific fan-made game about squirrels, but for veteran modders, it is the holy grail of base files.

: Played on PCs or smartphones via software like mGBA or VisualBoyAdvance.