Yes. The odds are initially set by professional handicappers and then go up or down depending on how much money is bet on each horse.
Somebody mentioned trifectas as betting on a pool of horses! Only partially correct, you have to pick them in the exact order they finish, exacta is two horses, trifecta three, in the correct order. Sometimes there are 4 or 5 horse options as a Quinella, but see those less these days. And there’s a superfecta which is 6, you can bet it for a dollar and get a huge payback but picking 6 in the correct order takes way more luck then handicapping skill.
Years back, you used to actually get a pool of horses. It was called " The Field". It was the 3 to 5 longest shots in the field lumped together and you could bet it to win, place or show. So if you took the field to show and bet $2 bucks on it and any of those field horses came in 1st, 2nd or 3rd, you got a payback. Haven’t seen that for years, not as many horses in the races as there used to be anyway.