Put a Tiger in Your Tank

The larval form of the tiger salamander (either eastern, western, or California) looks very much like the axolotl, the famous Mexican lake salamander with frilly external gills.  That's no surprise - the animals are all very closely related.  Axolotls are an example of neoteny, the retention of juvenile features in adult animals.  They are, essentially, amphibian Peter Pans who never grow up... that is, unless you give them a hormone shot, in which case you'll soon wind up with an animal that looks very much like a tiger salamander.  

A metamorphed axolotl, picture from Reddit.  Axolotls often have pink/albino coloration in captivity, but if this was a brown, wild-type individual, it would probably look much more similar to a tiger salamander.

The reverse can happen as well - sometimes tiger salamanders do not complete metamorphosis and stay neotenic, retaining their gills and aquatic lifestyle, even as adults.  Interestingly, these "big babies" may be larger than tiger salamanders which do undergo metamorphosis!

So, you could go to a zoo and see two salamanders in side-by-side tanks, one aquatic with large external gills, the other terrestrial and lacking those gills, and actually not be immediately sure which is the tiger salamander and which is the axolotl (not that I've ever seen such a display.  Nor what I recommend it, either - from what I understand, forcing an axolotl to undergo metamorphosis shortens its lifespan considerably, so best to leave things as they are.   A good educational exhibit isn't always compatible with optimal animal welfare, so I try to err on the side of the later. 

Eastern tiger salamander larvae, from USFWS