8.0.4 UPDATE
Tiktaalik ('tick-ta-lick', from the Inuktitut word meaning "Large freshwater fish") is a small carnivorous creature that was added in the 8.0.1 build. Commonly known as a 'fishapod' or the 'legfish' due to its peculiar limbs, it is a amphibian-like lobe-finned fish that lived in the Late Devonian period. They are diurnal, meaning they will be active at day and sleep at night. In real life, Tiktaalik represented a "transition" between fish and life on land.
A fossil version of the tiktaalik can be created by right-clicking a bio-fossil on the ground, which will create a random skeleton of a prehistoric creature with a small chance of it being a tiktaalik. Tiktaalik grows fully in 5 full Minecraft days. They can be given essence of chicken to grow faster at the cost of hunger.
BEHAVIOR
Tiktaalik are semi-aquatic, which means they can stay in both water and land. They are pretty slow on land, but fast in water. They prey on most small mobs. They will not play with bubble blowers.
TAMING
Tiktaalik, much like Meganeura, is not tamed on hatching, but needs to be tamed by giving it meat or using a whip on it. Tamed tiktaalik are ordered by right-clicking with a nautilus shell, and cannot be ridden.
FEEDING
Tiktaalik is a carnivore, meaning it eats all kinds of meat (Except rotten flesh). They will eat meat from feeders.
TRIVIA
Neil Shubin, the paleontologist who co-discovered the tiktaalik, tweeted about the mob being in the mod, linking this page and adding the caption "Finally....A Minecraft Tiktalik."