One of the first things you will figure out real fast playing Red Dead Redemption is that you have to be able to identify horses. Just like you have to know your mechanized transportation in Grand Theft Auto games, Red Dead Redemption is no different. The easiest way to know what breed a horse is, is not all that practical - buy it from the general store. This is bad not only because you are dropping coin on it, but what about horses that you acquire by other means?
The first big tip is to use the general store to find out horse breeds - just don't buy them. If you go to the general store it operates a lot like stores do in the real world in that you get a glimpse at what you're buying. Granted pictures would be great, but you can look at deeds for whatever horses are available for sale, and those deeds have descriptions of horses. You can jot those down if you have a bad memory - or for future reference - and figure by the physical description pretty much what any horse is when you break them.
A second way to determine a horse's breed is how they react when being broken. A one star horse for instance is going to drain stamina really fast, so that is a great way to start pegging a horse's breed. Similarly, a horse with stamina for days is going to be a three star, and one with moderate stamina falls in the middle as a two star. You'll also notice that the horse's speed will be commensurate to it's star rating. Just as cited above, a one star will not be all that fast, a two star will be middle of the line, and a three star flat out books.
Check out the horse's body for any indications as to it's health. If a horse looks like crap - it likely is crap. if you can see a horse's ribs that is a surefire one star horse. If it appears to be in moderately good shape but doesn't have the "wow" factor, odds are you're staring at a two star horse. If you encounter one that is well muscled and looks like a stud that was taken care of, then you have a three star horse.
Getting down to some specifics, there are 18 breeds - as best as I can find - that you will encounter at some point or another. Some are limited to being available in specific places or under special circumstances. There are three horses that fall into that category: The 3 Star Pearl, the Standard Breed Pinto, and the Hungarian Half-bred. The Standard Breed Pinto is only available in Mexico and will be all black. The 3 Star Pearl can be gained from doing the "Bonnie Mission." The Hungarian half-bred, like the Standard Breed Pinto, is only found in Mexico, and will be all white. In general, if you get a hold of an awesome horse in Mexico that is black or white, it is likely the Pinto or Hungarian Half-bred. If you get the Hungarian, try to complete the special trophy mission "Heading South on a White Bronco."
The Red Dead Redemption horse breeds you can encounter anywhere are presented in alphabetical order for ease of reference:
* American Standardbred
* Ardennais, Cleaveland Bay
* Dutch Warmblood
* Highland Chestnut
* Kentucky Saddler
* Painted Quarter Horse
* Painted Standardbred
* Quarter Horse
* Tobiano Pinto
* War Horse
* Welsh Mountain
Between information you can glean from deeds at the general store, the way a horse responds when being broke, a few distinct physical characteristics, and sometimes location, you can identify red Dead Redemption horse breeds pretty easily. Keep some notes on physical traits from the general store deeds to speed up the process - that really does help commit them to memory. It may seem tough at first, but the more you play, the quicker you get it.