If I use Guzzlord GX's "Eat Sloppily" attack and discard a Beast Energy 
card, would it get attached to the Guzzlord GX, or would it go straight to the Lost Zone because it was discarded?
When "discarding the top 5 cards of your deck", any revealed Energy cards get attached to Guzzlord GX immediately, before they reach the Discard Pile. Therefore, Beast Energy gets attached to Guzzlord GX rather than going to the Discard Pile or Lost Zone.
Eat SloppilyBeast Energy {*}