Some people go by a feeling, or the horse’s name, something they feel is lucky or meaningful in some obscure way. Some look at the odds. Some look at who the jockey is. Some look at the horse’s father and mother (bloodline) and whether there were any winners there, and the horse himself or herself, what their record of wins and losses are, some horses do better on a loose track, some do better with longer races, some do better with shorter races (come out of the gate quick, but tire in a longer race). All kinds of things.