Just the hidden ones. Let's not make this an Bulbapedia copy.
Don't know if they are there, obtainable hidden ones. I think there are, but that's what I will figure out.
Update:
I figured it out. 42 Pokémon are hidden, 40 non-obtainable and are listed by @Toupi.
The 2 other hidden ones are "Mewtwo" & "Rayquaza". They are not listed because they are obtainable.
The 3th Pokémon with "Catch but not keep"-mechanics is "Zekrom". He is not hidden on the Pokédex.
So no list needed other than Toupi's.