It's name is Nokk. And he or she is water spirit.
The mythical water spirit, the Nokk. Takes the form of a horse with the power of the ocean in the charge of a stallion The Nokk is a fierce warrior and guards the secrets of the forest fiercely. In order to get past the Nokk, a person must prove they are worthy and earn its respect - an almost impossible task.
Now we have an explanation of the reason why Elsa tried so hard to cross the sea in Frozen 2 trailer. She must earn respect of the Nokk.