Why are dolphins considered Ungulates?
Dolphins are considered ungulates because they shared a common ancestor with artiodactyls roughly 60 million years ago.
Dolphins are considered ungulates because they are closely related to artiodactyls (even-toed ungulates). Cetaceans evolved from an even-toed ungulate ancestor. Some have argued that the mammalian order artiodactyls needs to be combined with cetaceans to form the order, Cetartiodactyla, reflecting the close relationship between these two groups. This change is widely accepted.
Cetaceans are actually more closely related to even-toed ungulates than odd-toes ungulates (horses, rhinos, zebras) are.