A question on the timing of Genesis Hydra's Cast ability. If X >= 5 and the player finds a Garruk's Packleader, and puts that into play, then the Hydra come into play and the player draws a card off of Packleader's triggered ability?

So, how much like cascade is this?

That is correct.

It works this way because both for cascade and for Genesis Hydra the trigger condition is casting (and not “enters the battlefield”). So the trigger goes on the stack on top of the initial spell, and so resolves first.

Daniel Kitachewsky
L4, Paris, France
Rules NetRep