Is there a card that lets you put a card from outside of the game into play?
This is the "Wish" card series that were released in the Judgment extension.
For example: Burning Wish. It allows one for example in a tournament to get a card from the sideboard on the first game.

The 5 original wishes are:


Cunning Wish
Burning Wish
Death Wish
Living Wish
Golden Wish


Since then, other similar cards where released. This query should give most of them: o:"own from outside the game". You'll notice Ring of Ma&#39;r&#251;f in the results as well; the wishes are way more common.

So this is nothing "funky" like the fun cards in Unglued or Unhinged, and has been tournament-legal (and still is in some formats).

However these cards don't work by default in Commander/EDH except with prior agreement, so it sounds like your scenario wasn't how things should happen. See Commander rule 13:


  Abilities which refer to other cards owned outside the game (Wishes, Spawnsire, Research, Ring of Ma'ruf) do not function in Commander without prior agreement on their scope from the playgroup.


This means these cards shouldn't come out as a surprise, because beforehand you have to discuss and agree how they even work.