This subject is one of great confusion! If you search for the scientific names of Dwarf Puffers, you will come up with names like: Tetraodon travancoricus, Monotreta travancoricus and Carinotetraodon imitator.
I eventually learned that there are only two species (or variants): Carinotetraodon travancoricus and Carinotetraodon imitator. This information was provided to me at the Pufferfish Yahoo Group in two particular posts:
LinearChaos writes - "Ebert's book has all three of the dwarf puffers under Carinotetraodon...Carinotetraodon travancoricus (for the usually seen dwarf and the blue green) and Carinotetraodon imitator.
I've never heard of the others and I would stick with Carinotetraodon as a constant for the dwarf."
RTR writes - "To my understanding, the dwarf puffers (both species or both variants, whichever they turn out to be) should be called Carinotetraodon, as they have the keel/crest which is a determining characteristic for Carinotetraodon versus Tetraodon. Most taxonomists consider Monotreta a junior synonym for Tetraodon."
Below are photos of the male and female of each type. We are not quite sure if they are actually two different species: