If you read that title and went "WTF, mermaids?!", there are a few things wrong with your way of thinking. The original sirens were avian and later became mermaid-like creatures. The mermaid idea has snuck into several scientific terms including a usually fatal syndrome that merges a human baby's legs together (Sirenomelia). Also, manatees and dugongs are in the order Sirenia; supposedly, some extremely drunk sailors thought that they resembled human women with piscine tails.
EXTREMELY drunk. I want what they had. Alas, this is NOT the most perverted manatee joke I could make.
Great though manatees may be, this is not about them. There are far lesser-known amphibians that are also called sirens (family Sirenidae). Unfortunately, they sing about as well as Britney Spears and are nowhere near as sexy.
On second thought, that tail doesn't look half bad...
When was the last time you saw a tetrapod with two forelimbs that were definitely not fins and no hind limbs to speak of? If you said "naga porn," good for you; you have a VERY DIRTY MIND to which I can relate.
Sirens are native to the Southeastern U.S. and Mexico. Even though those limbs look stubby, like other salamanders, sirens can leave the water if the ground is wet enough. Similar to the axolotl, sirens retain their feathery juvenile gills. (Their juvenile gills are comparatively useless.)
Sirens can breathe through both gills and lungs. They are one of the most ancient groups of salamanders and boy does it show. They share the burrowing habits of lungfish. When they hunt, they inhale their prey. It's like a lamprey did it with a lizard.
More on sirens here. They're cool creatures!
...I should really do lampreys sometime.