From The New York Times:
Marmosets, small monkeys that live in South America, have long been a genetic enigma. Marmoset mothers almost always give birth to fraternal twins, which develop from two eggs and are thus genetically distinct. In 1962, scientists at Dartmouth Medical School discovered that almost all marmosets carry some blood-generating stem cells that began in their twin sibling.
Animals that carry cells from another individual are known as chimeras. Aside from marmosets, chimeras have been discovered in humans, cats and cows. But scientists have long thought that chimerism was a rare fluke.
Marmosets were different. Almost all of them had chimeric blood, and they were all healthy. It appears that they swap cells so often because of their peculiar development. In the womb, their placentas grow quickly and fuse, creating a network of blood vessels through which cells can travel from one twin to the other.
More here.