So is Regigigas like a species? Where does this question come from you ask? Well I was gonna add some stuff to Kane's Bio on The Rival Trainers, specifically the Pokemon he caught in each region. 

So I started thinking about the OG Regi pokemon: Regirock, Registeel, and Regice. Remember when these three Legendary Pokemon were supposedly Legendary Pokemon? The pinacle of the the Pokemon world you need plot armor to see because they're gods in the Pokemon world? 

Yeah well I found out that some are a fucking SPECIES. Not an individual, a species that has GENDERS, can BREED, and have CHILDREN. 

Where does Regigigas fit into this? Well turns out old Regigigas is the same way. In the Bulbapedia, I did some research and found a few instances of explaining:

"In the beginning of Hoopa and the Clash of Ages, a Regigigas was summoned by a Hoopa Unbound in Dahara City one hundred years prior to the events of the movie, alongside a Reshiram and Zekrom. The three fought Hoopa, but were easily defeated and sent back through portals to their original homes."

"In Pokémon Diamond and Pearl Adventure!, Hareta captures Regigigas by persuading it to help him stop Team Galactic. It first appeared in A Novel Test!!." Then they refer to it as "Hareta's Regigigas".

"In Chasing to the Finish!, a Dynamax Regigigas was awakened in the Crown Tundra after Project Mew succeeded in gathering all the legendary giants together."

Oh, and that set of Regi pokemon used in the anime are referred to as "Brandon's Legendary Giants", implying that there are more than one set. 

This isn't an angry rant, just a wild one.