The Vikings were an infantry force. Don’t think that they were transporting war horses on those longships in the pre-Christian era. That probably had to wait for the cogs and Spanish Caravelle.
I think if Vikings had landed on more fertile ground, so to speak, they would have brought horses over to expand their gains.
Not sure if this is correct, I just did a quick google search.
The Vikings transported horses overseas in boats very similar to Viking longships, but with flat flooring built within the hulls, which allowed the horses to stand. When horses were transported by boat, very large ships were used to support the weight of the horses, as well as provide more stability during the journey, so that the horses would not break their ankles.
Horses were lined up in the center of the boat, where they would experience the least amount of rocking from the waves, would be most helpful in keeping the boat balanced, and least likely to fall overboard. Viking ships were extremely stable vessels, and the shallow hulls allowed great versatility- including the transport of horses.
It must be noted: the horses which were transported overseas were highly trained calvary mounts. The horses had already been taught to remain calm and ‘steady,’ under chaotic conditions, and, to stay on their feet while being pushed around on challenging terrain during battle- this was an absolute must for a successful calvary campaign.
Furthermore, Norwegian Fjord Horses- the wild horses native to Scandinavia & the horses transported overseas by the Vikings- are notable for their physical sturdiness and stability. After all, the Norwegian Fjord Horse had evolved and adapted to living on the mountainsides of Scandinavian fjords- which is very dangerous terrain for any people/animals, who are prone to stumbling, or have very delicate ankle structures.
https://www.quora.com/How-did-the-Vikings-transport-horses-over-the-sea