Other than X&Y, Houndour and Houndoom have obtuse availability in every game they appear.

Kanto: Only appear after a timeskip between games, probably invasive
Johto: -
Hoenn: Safari Zone (foreign)
Sinnoh: Only available via pokeradar (rare foregn mon locator) or in the 3rd version (probably invasive)
Unova: Swarm (foreign)
Kalos: Available on a normal route right away
Alola: only available in the 3rd versions (probably invasive)

Houndour/Houndoom are Kalos pokemon and were introduced 4 gens early.

Maybe someday we'll find out where Corphish and Yungoos are from