Final answer:
Each of the horses ate a flake of hay by itself.
Step-by-step explanation:
When determining the correct reflexive pronoun to use with an indefinite pronoun, you pick the one that corresponds in number and gender, when gender is known. In this case, the correct reflexive pronoun would be 'itself' since each horse is eating a flake of hay independently and horses are generally referred to as 'it' unless their gender is specified.
Reflexive pronouns are used when the subject and the object of the sentence are the same entity, and they always end in self or selves. It is important not to confuse reflexive pronouns with personal pronouns or mistakenly use them to sound more formal.