Yes, there are other hybrids that occur in animals, but before we go too far, let's clear up a couple of terms. Mules are not a species since like most other hybrids they can't reproduce. In fact, the term "hybrid species" is a bit of an oxymoron. Basically, a hybrid is the result of crossing two species, and is not itself usually classified as a separate species. A hybrid would only start to be recognized as a separate species if the hybrids could be bred to reproduce offspring like themselves and distinct from either parent species, although that would only be the first step. (This is a bit oversimplified in terms of taxonomic nomenclature, but that's another issue.) Another interesting point is that breeding a male of species A to a female of species B often produces something different that breeding a female of species A with a male of species B. For instance, a mule is specifically the offspring of a male donkey and a female horse. If a male horse is bred to a female donkey, the result is a different hybrid called a hinny. In the animal kingdom, hybrids are often sterile due to differences in the number of chromosomes in the parent species. Although there have actually been very rare cases of female mules that produced living offspring when bred to purebred horses or donkeys (a total of about 60 documented cases in the last 450 years or so) and one documented case of this happening with a female hinny, both hybrids are considered sterile as is most often the case. Lions and tigers produce hybrids in much the same way. The offspring of a male lion and female tiger is called a liger. Ligers apparently look like giant lions with diffuse stripes and unlike their lion fathers like swimming. On the other hand, the offspring of a male tiger and a female lion is called a tigon. This is a rare cross that reportedly doesn't occur in the wild. Although tigons and ligers are the best known crosses in the panthera family, there are names and information about many other possible crosses that can be found at http://en.wikipedia.org/wiki/Panthera_hybrid Perhaps one of the most commonly known hybrids in North America is the "hybrid wolf" that occurs when wolves are bred with dogs. Wolves and dogs are different species but will interbreed readily because they have the same number of chromosomes. There is no difference between the offspring of male wolves/female dogs and female wolves/male dogs. While the hybrid offspring are fertile, they are not considered a separate species. These hybrids are often unstable and dangerous because the wolf traits often breed true, resulting in a "pet" that doesn't act or respond the way dogs do, often with unfortunately consequences for the owner and others. (You can read about some of the differences between wolves and dogs at http://www.answerbag.com/q_view.php/39319) While these are the best known of animal hybrids, there are actually many such oddities including the zeedonk, beefalo, wolphin, and cama. You can read more about some of them at these sites: http://en.wikipedia.org/wiki/Hybrid#Hybrids_in_nature (Scroll down a little way for a list with links.) http://abcnews.go.com/Technology/popup?id=681089&content=&page=1 (Includes pictures of hybrids.) http://www.greenapple.com/~jorp/amzanim/crossesa.htm (A personal page with interesting information, links, and a pretty amazing collection of photos.)