Stybba was a pony of Rohan. Described as shaggy and grey and suitable for mountain riding but slower than the horses of the Rohirrim.
When Aragorn, Legolas, and Gimli rode off Hornburg with the Grey Company, King Théoden lent to Meriadoc Brandybuck Stybba. Stybba carried Merry through the mountain passes from the Hornburg east and south to Dunharrow and thence northward to Edoras.
Stybba was too slow to accompany the mustered army of Rohan across the plains, and Théoden ordered Merry to stay at the Courts with Lady Éowyn. Thus Stybba was left behind but Merry did secretly complete the journey with a Rider named Dernhelm (who was in fact Éowyn in disguise).
 Portrayal in adaptations
- Merry is indeed shown riding a small pony while traveling with the Rohirrim from Edoras to Dunharrow. Though this pony is not identified as Stybba, it is obviously a reference to Stybba.