Temple Grandin says domesticated dogs are wolf puppies that do not mature due to genetic selection. This is supported by brain MRI's of young and mature wolves.
Just thought I'd throw that in. Wolf puppies apparently are playful and willing to please, until they mature.
Not sure I'd want a horse as eager to please as some dogs who want to jump up on your chest and lick your face. Hee
Yes, it's called neoteny. It's fascinating -- in the famous Siberian fur fox breeding experiment, it was found that selecting for tameness and friendliness in fur foxes creates foxes that have white paws, noses and chests (ruining their value as pelts). That's caused by incomplete maturation, as those areas are the last to be pigmented in utero.
Grandin isn't exactly right though; dogs are a separate species from wolves, which split off some 40,000 years ago. They evolved to be scavengers in human encampments. In order to tolerate and be tolerated by humans, they did what the foxes did -- stopped maturation early. This was not selection by human beings but rather, the tamer wolves were able to get in closer, ingratiate themselves with humans, so got more food and survived to pass on their genes. They also have smaller brains and teeth -- didn't need those for group hunting any more.
So next time you are trying to decide whether your dog should be eating canned buffalo and sweet potatoes, or raw meat and ground up vegetables which mimic the diet of wolves, remember that for 40,000 years dogs have been eating garbage, offal, and feces, and thriving.