There's no payout if the horse doesn't win. It doesn't operate like the stock market.
The odds reflect the horse's supposed chance of winning or placing (or whatever else it takes to get a payout), not its expected position -- so this isn't a case where you can be aware that the horse is underrated, but have no way to make a +EV bet on that knowledge.
The first order of business in these things is certainly to spot the overlay/value pick/+EV situation. The second order of business is to manage bankroll/vol to be able to realize the EV over time -- but that is generally speaking much easier and more mechanical than the core insight of (repeatably) finding the EV...