In Disney's Frozen II, the water spirit that takes the form of a horse is the Nokk.
The Nokk is one of the elemental spirits connected with the enchanted forest and the waters surrounding it. Elsa encounters the spirit while attempting to cross the dark sea. It first appears as a forceful, horse-shaped presence made from water, challenging her ability to control and understand the natural elements.
After Elsa proves herself, she establishes a connection with the Nokk and rides it across the sea. The film presents the Nokk alongside Gale, the wind spirit, Bruni, the fire spirit, and the Earth Giants. The name is drawn from Scandinavian folklore, where water spirits known by related names are often associated with horses and dangerous bodies of water.