They mutate into the creature on top plus all abilities from under it.) Whenever this creature mutates, other creatures you control get +X/+X until end of turn, where X is the number of times this creature has mutated
The Pokmon-GX rules box took the place of the Pokdex entry
This could be anything, ranging from putting a +1/+1 counter on a creature, exiling cards off the top of your library, or even playing a permanent for free
So take a look at your old collection and see if you have a little nugget of gold stashed away
In the incredibly rare case where there is no legal target for the first or second mode, those modes can't be chosen at random
If you do, search your deck for up to 2 basic Energy cards and attach them to the Pokmon you evolved in this way