When Zangoose and Seviper were added to Pokémon Go in a late 2017 Gen 3 update, each one was exclusive to half of the world. Here in the UK we got Seviper, while America got Zangoose. The reason no region got both Pokémon at once is that these Pokémon are notorious rivals in the Pokémon games and anime — one's a mongoose trying to eat the other, a big snake. It's more or less how things play out in the real world.
Interestingly, as the recent Christmas event just ended in Pokémon Go, Seviper and Zangoose seem to have switched their regional exclusivity. It's unclear how long this switch will last for, but currently UK players can get a Zangoose if they go hunting, with a similar level of rarity to the previously present Seviper.
In terms of upcoming expected Pokémon rotations, it's expected Kyogre will be the game's next legendary raid, starting around January 15th when the Groudon raids end.