From The New York Times:
In 2006, a hunter in the Canadian Arctic shot a bear that had white fur like a polar bear’s but had brown patches, long claws and a hump like a grizzly bear’s. DNA analysis confirmed the animal was a hybrid of the two species.
While one might think that these oddities are examples of some kind of moral breakdown in the animal kingdom, it turns out that hybridization among distinct species is not so rare. Some biologists estimate that as many as 10 percent of animal species and up to 25 percent of plant species may occasionally breed with another species. The more important issue is not whether such liaisons occasionally produce offspring, but the vitality of the hybrid and whether two species might combine to give rise to a third, distinct species. While several examples of human-bred animal hybrids are well known and can thrive in captivity including zorses (zebra-horse), beefalo (bison-beef cattle) and, of course, mules (donkey-horse), naturally occurring animal hybrids have many factors working against their longer-term success.
More here.