The most commonly observed two-headed animals are turtles and snakes. Other species with known two-headed occurrences include cattle, sheep, pigs, cats, dogs, and fish.
When you see a two-headed, or polycephalic, animal, you're actually looking at a set of twins that failed to separate after fertilization. This can result in the conjoining of twins when a gene known as "sonic hedgehog" or "SHH" is increased during embryonic development, which can cause the head or face to widen, often resulting in two faces. When a group of cells known as organizers becomes triggered, two entirely separate heads can form, resulting in individual necks stemming from a single body.
Now, if you finished to read my comments, what many people in a hurry don't do: in my pic, there are really 2 birds, one behind the other...not a two-headed parakeet!