Why mutations in animals but rare in humans?

In the news every once in a while on the news there is animals with 2 heads or extra feet (like the recent duck) or frogs with arms growing out of their back. But not with humans. I know joined twins happen where the egg does not split when two sperm get in but thats not a true mutation (I'm not saying joined twins are mutants) I know there is an actor or some one famous with 3 nipples or something but i have never heard of a human with 2 heads or 4 legs. I know that there are many many more animals then humans so the odds of it happening with animals is a greater chance then humans but is there another reason? And do the mutation parts move with with brain activity, like a duck with 4 legs really be able to walk with all 4, i know the news link said "...he is running about with those extra legs acting like stabilizers" but can he control them?

http://news.yahoo.com/s/ap/20070219/ap_on_fe_st/fo...