https://www.polygon.com/pokemon-go/2017/12/29/16830184/relicanth-pokemon-go-gen-3-regional-exclusive

To find one of Pokémon Go’s new region-exclusive Pokémon, you may have to take a vacation. Members of The Silph Road subreddit have found substantial evidence that Relicanth, one of the rarest Pokémon introduced in the game’s third-generation update, can only be found around a handful of countries in the Pacific Islands.

Players have reported finding Relicanth in just a few areas: Fiji, New Caledonia and Vanatu, as well as New Zealand, the most populous of the four. With just around six million people living in these countries combined, not many Pokémon Go trainers have seen the coveted water- and rock-type.