Hello
According to the PokéDex, Tentacruel is available underneath the Grand Stairway. I checked the Pokémon locations guide and the Training Guide, but they say that it is only available through the mistery egg.
I have searched beneath the stairs for Hours, without luck, but can someone confirm that it is under the stairs or is it only the mistery Egg? If it is only the mistery egg, I think its (tenta)cruel to let people think its there.