In any given horse race the odds are determined by what is called a Pari-Mutuel system. Which is just a fancy French term for mutual betting. It means that the odds are determined by us- the betting public. When you look at a track program you will see odds listed for every horse that is scheduled to race that day. But those are just the Morning Line odds that have been set by the track handicapper. And while they are usually fairly accurate, they are just a guide, a jump off point, which in a lot of cases have no impact on the final odds on a horse. In actuality the odds are set by the amount of money that is bet on each horse, relative to how much is bet on the other horses in the race. The more money that is wagered on any particular horse will lower the final odds on that runner.
Because the odds are determined this way, they will fluctuate throughout the betting period prior to the race. In some cases the odds are likely to change drastically. It’s not uncommon to see a horse that was a Morning Line long-shot become a Post Time favorite, due to the fact that someone (or a lot of people) made a large wager on him/her when the betting on that race began. Conversely, there are horses that begin with low odds, but do not take a lot of money (be bet) and go off at much higher odds than set by the track handicapper. This is where the Daily Racing Blog looks for value; when a talented horse should be getting bet heavily but is not, and the odds are a bit out of whack. CATHOLIC BOY in the Travers Stakes is a great example. A very talented horse, had won on both dirt and turf, always in the money, and inexplicably going off at 8-1 odds.
The take-away here is that your payout, most likely, will never be based on the odds when you placed your bet. The payout will be based on the odds when the race begins. This is different than sports betting, where the odds are locked in when you make your bet. Because of the pari-mutuel system it is important (when in search of a value horse) to be aware of what the Morning Line odds are on any given horse and try to get an idea on what they might be at Post Time.