It's probably the Dark Rattata mentioned in the rumor that first mentioned Variants.
The Asian mongoose was brought to take care of rats, but I believe the rats were nocturnal and thus the mongoose didn't do that right. Probably something like this in Pokemon...
Yungoos comes in and takes care of the Rattata during the day, so they evolve to only come out at night when it's safe, maybe becoming more sneaky and crafty so Dark-type. Now that's why Gumshoos is mentioned to spend all day staking out but always fails, the Alolan Rattata know to come out at night.
Dark Rattata is probably on Route 1, but only at night.
