https://www.polygon.com/2018/8/17/17718190/pokemon-go-regirock-raid-battle-dates

Pokémon Go’s summer of the Regi-trio — Regice, Registeel and Regirock — is coming to a close. Raid Battles featuring Regirock, the rock-type legendary Pokémon native to the Hoenn region, are now in-game through mid-September.

The rock-type (duh) Regirock takes the place of Registeel (a steel-type), the most recent timed-exclusive legendary to post up at gyms worldwide. Neither Registeel nor Regice (guess which type it is?), the first of these legendaries to enter Pokémon Go, drummed up much fanfare. Pokémon Go’s meta doesn’t have much of a need or place for the group, and capturing them in Pokémon Go is less interesting than the challenge to catch them in their debut games.