Eternal Dragon

• WOTC Official Reminder Textofficial rules
  Cycling — Cycling [cost] (Pay [cost], discard this card: Draw a card.)

• WWizards of the Coast2009-02-01
  Official Ruling
  Unlike the normal cycling ability, Plainscycling doesn't allow you to draw a card. Instead, it lets you search your library for a Plains card. After you find a Plains card in your library, you reveal it, put it into your hand, then shuffle your library.
• WWizards of the Coast2009-02-01
  Official Ruling
  Plainscycling is an activated ability. Effects that interact with activated abilities (such as Stifle or Rings of Brighthearth) will interact with Plainscycling. Effects that interact with spells (such as Remove Soul or Faerie Tauntings) will not.
• WWizards of the Coast2009-02-01
  Official Ruling
  You can choose to find any card with the Plains land type, including nonbasic lands. You can also choose not to find a card, even if there is a Plains card in your library.
• WWizards of the Coast2009-02-01
  Official Ruling
  Plainscycling is a form of cycling. Any ability that triggers on a card being cycled also triggers on Plainscycling this card. Any ability that stops a cycling ability from being activated also stops Plainscycling from being activated.

Rulings via Scryfall