The tiger salamander is one such critter:
http://www.npwrc.usgs.gov/narcam/idguide/atigrin.htm
Can be a nasty predator and cannibal or a docile aquatic thingy, depending on the environment. Probably not what you were referring to, but similar, no doubt.
Thanks furball! The tiger salamander isn't what was the subject of the recent documentary I saw, but interesting nonetheless. This was some sub-saharan creature totally dependent upon water, or lack thereof. If it is a dry year, the creature lays eggs that turn into an amphibious animal. In a wet year it lays eggs that turn into an acquatic animal. I think this was on Discovery or Animal Planet. I'll see if I can find it. It's interesting because drought could render this species extinct but somehow it adapts to either wet or drought conditions during egg laying. So how do it know?? Maybe if it rolls snake eyes, we have X kind of eggs? What if it layed eggs for drought conditions and then the monsoons set in. Or layed eggs for wet conditions and a drought set in? No intelligent design here, nope.