When the sun goes down, it becomes active. It runs around town on a chase for good food for the boss of its nest—Raticate.
– Pokémon Moon Pokédex
Alolan Rattata is Dark/Normal instead of Normal. For some reason, Dark is its primary type, not Normal. It’s one of 3 families with Normal as its secondary type.
Like its Pokedex entry says, Rattata only appears at night.
Alolan Rattata has the same stats as Kanto Rattata; it has good Speed and Attack.