Question
I have noticed an unusual looking critter in our larger freshwater pond.  NW Washington state.  It appears to come up for air rarely.  It doesn't stay at the top but a few seconds.  It appears to have a hammer-head with a type of fish tail.  Possibly 3-inches in length.  I have noticed smaller.  So far it has been too quick to photograph.  Any ideas?

Answer
It sounds like it is simply a Tadpole - which is a "baby frog" once they get larger the heads look very large which is really the body bulking up to grow legs. It will soon grow larger and have no tail, and grow legs and then hop out of the pond. It should pose no harm to your pond and could help control pest insects that could harm your fish.

Here is a photo of a tadpole with a "hammer" like head and "fish tail" http://www.lancing-nature.bn15.net/natimages/amphibian/tadpole9628.jpg