Fishmask

The Fishmask is a temporary reward from the Squeal of Fortune. It is only obtainable during the 4.6-10.6 as an uncommon prize and afterwards as a rare prize throughout the summer months of 2012.

It is tradeable, and will probably become a rare item after some time, because it is very hard to get a rare prize on the Squeal of Fortune. Jagex probably made the mask tradeable so that more people would buy spins or membership for RuneScape.