Horses and Ponies are mentioned on multiple occasions in the How to Train Your Dragon Book Series.
Horses (Equus ferus caballus) are four-legged animals that have been domesticated for at least four millennia. The original truly wild horse is extinct, except for the endangered subspecies, the https://en.wikipedia.org/wiki/Przewalski%27s_horse Przewalski's Horse]. There are a large number of horse breeds originating from many different locations and cultures. Small horse varieties are generally referred to as ponies.
Historical Vikings were well versed with horses and used them for many purposes. Two breeds in particular have remained relatively genetically pure since the Viking Age with little influence from outside breeds. These are the Fjord horse and the Icelandic Horse. Both, whether called a horse or a pony, depends who you talk to. The Fjord horse is small, stocky, and has a thick well-muscled neck. They are sure-footed and always have a dun-colored coat. Fjords' manes are almost always "roached" - trimmed - away from the neck. They have a good temperament and are used for a variety of activities. The Icelandic horse developed alongside the Vikings that inhabited Iceland and remained isolated. Icelandics are also small and stocky, with bushy manes. They are hardy and long-lived. Icelandics come in a variety of colors and are also a multi-purpose breed.
In the Book Series, horses are mentioned as a beast of burden, an animal inferior to dragons, and as a creature edible to dragons.
In reality, horses are indeed used for transportation and to carry things or pull carts. Horses are also used in a variety of equine sports and for other entertainment. They are eaten by humans, especially in antiquity.
Gobber the Belch first mentions horses when addressing the Viking novice class.
|“||Lesser humans train hawks to hunt for them, horses to carry them. It is only the VIKING HEROES who dare to tame the wildest, most dangerous creatures on Earth!||”|
|“||However, a dragon is not a fluffy creature like a dog or a cat or a pony."||”|
Horses are later mentioned as a meal for a Seadragonus Giganticus Maximus.
|“||He'd had the luck to catch a Roman Legion camping on a clifftop - they were completely cut off and he had spent an enjoyable afternoon wolfing down the whole lot of them, from commanding officer to lowliest private. Horses, chariots, shields, and spears, the entire lot went down the ravenous, reptilian gullet.||”|
|— How to Train Your Dragon|
Unlike the Vikings, the Romans rely heavily on horses for ground transport and work. At the Roman outpost, Fort Sinister, there are "exercise yards for the horses" and the fort was "busy with soldiers and cooks and horses and people selling things to each other." Artwork on the side of the Fat Consul's bathtub also contains a depiction of a horse.
|“||Indeed, its whole body seemed boiling hot, and like the flanks of a horse that has galloped for miles, great clouds of smoke rose up from its tremendous gleaming bulk and into the sky.||”|
|— Book 4|