Which descriptor would be appropriate for the juvenile stage of horses?

The descriptor "foal" is appropriate for the juvenile stage of horses because it specifically refers to a horse that is less than one year old, regardless of its sex. This term encompasses all young horses of that age group.

The other options refer to more specific categories within the juvenile stage: a "colt" is a male horse that is typically younger than four years old, while a "filly" is a female horse of a similar age. "Poney" does not accurately describe a juvenile horse, as it refers to a distinct type of small horse breed rather than a stage of life. Using "foal" ensures clarity in identifying the youngest class of equines.