I'm with you so far Princess; same as dogs so far. Except as I said once before, with dogs you do get 10 or so in a litter and, in a pedigree as mixed as we seem to get in thoroughbreds, the range of puppies would be quite wide, with a few up to show standard and some clearly not. With the litter of course one can take your pick as to which one(s) you keep for yourself.
The main difference seems to be that, in dogs, a bitch produces several eggs, each containing a combination of genes from her parents, grandparents etc, whereas in horses a mare only produces one egg (occasionally two), one of which is terminated. So the element of choice is non existent.
The chances of an even litter in dogs (and hence relatively significantly greater predictability) is increased with careful line breeding (the occasional necessary outcross I've covered in more detail somewhere*) and is the route to successful breeding in dogs. I may be wrong but, with horses, it seems to me that line breeding is the exception rather than the rule which, also to me, would seem to explain why it is so difficult to predict what a sire/mare combination might produce. If horses are anything like dogs, then the more "choppy" the pedigree the greater will be the element of chance and, without the element of choice, is a very risky business.
I hope that makes some sort of sense.
Edit *
Found it. Sorry much is repeated