If your fair charges a fee per animal, instead of per class, we suggest you set up the fee as a Fair Level custom field, make it required, but give an "out" to those not showing that specific species.
Examples:
These fees will be added onto the exhibitor's invoice. Please keep in mind that the fair level custom fields are a one-time exhibitor question. Only the first invoice/registration will include the fair level custom fields/fees - once exhibitor is approved, they will not be able to edit or add that information to a later invoice.
A manager can edit the information, but it will not change the fees on the invoice, or create an additional invoice if there are additional fees--so any additional money will have to be documented outside of FairEntry.
This is especially important to note if you have an earlier registration period for a special event, like a communications contest. The first invoice will contain the question; other invoices created later will not.
See more about Custom Entry Fields: Setting up Custom Entry Fields