There isn't any Pokemon in the Abyssal Ruins.Only ancient items and Unown letters.
PS : When I first went there I thought I would find Kyogre or Arceus :P
There are only items in there, no Pokemon.
Bulbapedia Abyssal Ruins
There are many hacked Abyssal Ruin Legendary Pokemon encounters
Here are two of them
You can only find items in the Abyssal Ruins Read the Bulbapedia Link for a list of the items you can find. There are some Secret messages inside the Abyssal Ruins