Can Zoroark's "Foul Play" attack use Medicham's "Yoga Loop" attack to repeatedly get extra turns?
Yes, Foul Play on Yoga Loop could work infinitely, if you're able to pull it off!
Source: TPCi Rules Team (2021-09-23)
What happens if Zoroark uses its "Foul Play" attack to copy Haxorus' "Dragon Dance" attack? Would it apply to Zoroark or not (because its name is not Haxorus)?
Zoroark's Foul Play can choose Dragon Dance, but since its name is not "Haxorus" the attack will basically have no effect. Dragon Dance specifically refers to Haxorus, so any Pokemon not named Haxorus cannot receive the damage boost.
Source: TPCi Rules Team (2015-10-29)
If Zoroark uses "Foul Play" to copy Blastoise's "Hydro Launcher" attack, do I have to be able to return 2 Water Energies to my hand in order to do the 100 damage?
No, the attack does not say "in order to" so you do as much of the attack text as you can.
Source: PUI Rules Team (2011-06-09)
Do I have to meet the energy requirements for the attack I choose with Zoroark's "Foul Play"?
No, you do not have to meet the energy requirements for the attack. However, if the attack says you must do something or the attack does nothing, that part still has to be done.
Source: Black & White FAQ (2011-04-28), PUI Rules Team (2011-04-28)
If Zoroark uses "Foul Play" to copy Reshiram's "Blue Flare" attack, what happens if I don't have have any Fire Energy attached to Zoroark?
The damage amount is done regardless of whether Zororark has any Fire Energy attached to it or not, but if you have one or two Fire Energy attached to Zoroark you must discard them.
Source: Black & White FAQ (2011-04-28), PUI Rules Team (2011-04-28)