It would seem that Pokémon GO's "spawn nests" have been entirely randomised for rare 'mon.
Reports are appearing online that Niantic has changed things up for hard-to-find monsters, deviating from its previous approach of simply switching one type for another at predetermined spawn locations.
According to Forbes, these "nests" now spit out a totally different set of Pokémon, with relative rarity being the only link. Previously, when Niantic had tinkered with spawn nests it had replaced one monster for another, so it was easy to track the changes and identify spawn locations for a particular Pokémon.
It's worth pointing out that this doesn't impact the low-level spawns that you find on a daily basis, so you'll still be running into plenty of of Weedles and Pidgeys on your travels.
Have you noticed that you're seeing different Pokémon where you live now? Has this change thrown a spanner in the works for any tactics you had for evolving a particular Pokémon or powering it up? Let us know what you think of the move by leaving a comment below.