Giant sea horses are used as mounts for the Sea People in Narnia.
Portrayal in adaptions
In the illustration by Pauline Baynes, they are simply large horse-sized sea horses.
Since the Sea People were both omitted from the film and replaced by naiads, the sea horses were not portrayed in the 2010 film either. However, concept art for the sea horses depict them as looking more like hippocampi in Greek mythology.