The Regional Variants Game

The Old Chateau in Sinnoh.

Skwovet (Bug/Ghost)

Ilex Forest, but only at night.

Skrelp (Dark/Fairy)

Underwater via diving in Hoenn

Furfrou (Water/Normal)

Olivine City

Chinchou (Water/Fire)

In the extreme depths of Hoenn's underwater, as far down as you can get. Pretend there's a hidden dive spot in Route 128 that goes very deep underwater. The waters are warmer there, and that's where you'd find water/fire Chinchou. :D

Manectric (fire)

Jagged Pass seems like a fitting location. Rugged, close to a volcano. Should feel right at home.

Altaria (Fairy/Normal)

A misty forest filled with Volbeats and Illumises. There's a house where of an old, retired witch adopted fairytale girls and boys. Upon entering her bedroom, the Old Hag gives you the following Altaria.

Steel-typed Azumarill, here we go. Neon lamp-inspired.

Ten Carat Hill, but only if you have a full party of minior, with each having a different color core.

Speaking of:
Minior (Psychic/Rock)

Sound like an event Pokemon at a shrine where it is worshipped as a holy stone.

Shellos (Water/Grass)

The patch of water just outside Mt. Silver!

Hypno (Dark)

Old Chateau. Better watch out next time you enter!

Tropius (Psychic/Flying)

Among the ruins of Kalos' Victory Road (the outside areas)

Machamp (Bug)

Hmm... Route 20 of Kalos, or called the Winding Woods.

Tentacruel (Water/Electric)

the water near Olivine City, drawn in by the lighthouse =)

Houndoom (Dark/Fairy)

Pokemon Mansion in Kanto. Provide the two types Kanto needs, possibly stray dogs among the ruins.

Golem (Ice/Rock)

Any Icy caverns, such as the Ice Path in Johto.

Grumpig (Dark/Fighting)

Any victory road, due to harshness of ecosystem.

Eevee (Ghost)

Either Mt. Pyre or Pokemon Tower, playing with it's ghost pals =3

Jellicent (Flying/Psychic)

In a desert where it causes Fata Morganas with its Psychic powers.

Metapod (Steel)

Residential areas like goldenrod city.

Pallossand (Ground/Rock)