There are a number of Bovine species which are referred to as 'buffalo'. It is unclear which species or group the author is referring to. There are three Genera which contain members called 'Buffalo': Bubalus, Bison, and Syncerus. The latter has only one species - the African or Cape Buffalo. This one is the least likely 'buffalo' the author may be referring to.
This Genus contains several species referred to as water buffalo, as well as anoas and tamaraws. The water buffalo (Bubalus arnee) has several subspecies that have been domesticated by humans for at least 5000 years, primarily in Asia. Water Buffalo were introduced to Mediterranean Europe around 600 AD, several hundred years before the Viking Age. Historic Vikings did trade across Europe and the Mediterranean, as well as the Near and Middle East, so may very well have encountered these beasts.
Water Buffalo have a 'standard cow' shape. They lack humps like bison or some cattle breeds. All subspecies are typically black, very dark grey, or shades of very dark brown. They also have a pair of horns growing out of the head slightly downward, then make a wide curve toward their backs. There can be variations in the horns - some have more curvature, some sit higher on the head and grow more upward. Some are thicker than others.
The American Bison (Bison bison) is commonly and popularly called the 'buffalo'. It is a tall, dark brown bovine with a thick shaggy coat on its head and forepart of the body. It has a pair of short black horns curving to the front. They have shoulders that sit well above the line of the head, especially in bulls. The American bison only occurs in North America, and was once widespread across the continent before the arrival of Europeans. In fact, Europeans had already begun destroying the population of their own native species, the European Bison (Bison bonasus), more widely called the Wisent.
The Wisent once lived in a range from France through Eastern Europe and possibly into Eastern Russia. Today, it is critically endangered and only a small heard lives in a forest at the Belarus-Poland border. There are, however, programs in various countries to reintroduce them. The Wisent looks similar to the American Bison with longer legs and less bulk. There are a number of anatomical differences however, such as different numbers of ribs and vertebrae.
Water buffalo have been hunted and raised for meat for several millennia. Humans also make use of their hides for leather and horns for various products. Like other large livestock, they are used as draft animals. Water buffalo are also raised for their milk. It generally has a higher fat content then milk from cattle. Italy in particular is know for its Italian Mediterranean buffalo breed, which is used for making buffalo mozzarella, as well as other cheeses.
The bison is hunted predominantly for meat, hide, and other parts.
|“||Toothless hopped down onto the banqueting tables and tucked into the roast buffalo pie, ignoring Hiccup's furious, frustrated whispers from the cage swinging a couple of meters above.||”|
|— Book 4|
Hiccup also later fought the Hysterics with their own food, "knocking out a Hysteric with a leg of roasted buffalo".