- "No, no, they're not real! They're monsters, they're sirens...don't listen to them! They may look real but they're not!"
- ―Manny to Sid, Diego and Granny about the sirens' disguises.[src]
Sirens were fish-like carnivorous animals that usually lay on rock outcroppings near the ocean, where they would try to lure in any creatures that got close by casting images of whatever they found most attractive, be it loved ones, an attractive potential mate or even, in the case of Scrat, the Saber-toothed squirrel, items, such as Scrat's prized acorn.
Medium-sized fish-like creatures, the sirens were bluish in hue and sported a number of fins which could be used to move on land as mudskippers would. The sirens had large, bulbous eyes and mouths filled with needle-sharp teeth, but perhaps their most pertinent trait was their ability to morph into whatever their target prey found most appealing, which could be controlled at will, though their disguises would sometimes falter somewhat.
Sirens resided in the shallows facing the oceans. A number of them resided near rocky cliffs and appeared before a group of animals: one mammoth named Manny, a saber-tooth cat named Diego and two ground sloths named Sid and Granny. The animals were aboard a great chunk of ice that was fashioned into a ship and which they were piloting towards their home. The sirens, wishing to attract the attention of the mammals, took on the forms of what they each found most appealing, desirable or beloved through the mist surrounding their shallows; first appearing to Diego as the saber-toothed cat named Shira, followed by another two appearing to Sid and Granny as a female sloth mermaid and a muscular male sloth, respectively. Finally, the last sirens appeared to Manny as his wife Ellie and their daughter Peaches.
The sirens, as they said and did whatever those they tried to lure found most appealing, appeared as Ellie admitting Manny was always right, which promptly convinced Manny that the sirens were not his wife and daughter, noticing then that the sirens were trying to lure them all towards the rocky outcroppings so that the rocks would destroy the ship. Manny steered the vessel away from the rocks, but the others continued to fall for the sirens until Manny managed to steer the ship away from the sirens in time, as one of them was about to attack Sid. The ship was steered away and the sirens called out after it, upset at having lost their quarry. Sid, however, called out to the siren that had tried to catch him, asking it to call him, but the siren, wanting nothing to do with the sloth, disgustedly declined.
At that moment, the sirens spotted another potential victim: Scrat, a saber-tooth squirrel, who was traveling on a small ice floe. At first one of the sirens appeared as Scratte, an old flame of Scrat's, but within a moment Scrat turned the vision down: after that, the siren appeared as Scrat's prized acorn. The projection worked too well as Scrat, in trying to bury the acorn he saw, stomped it down, accidentally killing the siren, leading the others to pretend to be acorns and chase him. Scrat got away and the sirens were unhappy again.
Further on, one of the sirens, detecting more prey, appeared before Gutt, a piratical Gigantopithecus, as a female of his kind, replete with a bounty of fruit and within a massive shell. The siren called out to Gutt as a female would, offering to rule the seas with the ape, but revealed its true form when Gutt got too close to the shell, grabbing the ape by the face and pulling him into the shell, which clamped shut. All that could be heard of the captain was calls of pain.
Behind the Scenes
The sirens were based off ancient Greek legends of humanoid sirens: these sirens would instead call out to whatever seafarers found their way near the rocks where the sirens lived.
When filming the sequence with the sirens, special care was taken with lighting and materials to cause them to appear much more gruesome. 
- ↑ http://peterdeseve.blogspot.com/2012/07/decade-has-now-passed-since-i-did-my.html Peter DeSève's Blogspot