In the EX:Sandstorm set, there are 2 identical versions of Marill, but one has a retreat cost and one does not. Which one is the correct version and how shall the 2 different versions be played?
This appears to be a fairly common mistake in the set. Play the card as written; if you have the Marill with the free retreat you may play it as such.
Source: PUI Rules Team (2003-09-11)

Relevant cards

Note: All applicable cards may not be listed below.
EX2_68