[G1] Episode 31-34 - "The Magic Coins" (1986)
The ponies are delighted to discover a treasure chest of magic coins, until Ponyland becomes a drought after they wished it stopped raining. To undo the spell, they must appease the troll to whom the treasure really belongs.
(Please, please please PLEASE support the official release. Buy the DVDs. I did, you should too. It's good stuff. Show Hasbro your support so they'll keep making more of these!)