The cetartiodactyls are a crown group. That is, they are defined as the last common ancestor of two living animals and all descendents of that common ancestor -- in this case, the last common ancestor of deer and dolphins. For that matter, it could be "whales and gnus," but that would be an utterly rotten thing to do. No reasonable language has both the English 'wh' and the palatal click ('gn') of Swahili. Most languages lack both. . But we have, as so often in the past, wandered far from our intended path.
The cetartiodactyls include a large number of familiar beasts, including whales, dolphins, pigs, hippos, cattle, deer and antelope. These are the artiodactyls or "even-toed ungulates" plus whales. Obviously the number of toes in whales is a somewhat moot point. Except for some early whales, most of the cetartiodactyl groups lack even minimal coverage in Palaeos at the moment, so we will defer most discussion.
-- ATW 020314
"G" stands for Gnu, whose weapons of Defense
Are long, sharp, curling Horns, and Common-sense.
To these he adds a Name so short and strong
That even Hardy Boers pronounce it wrong.
How often on a bright Autumnal day
The Pious people of Pretoria say,
"Come, let us hunt the --" Then no more is heard
But Sounds of Strong Men struggling with a word.
Meanwhile, the distant Gnu with grateful eyes
Observes his opportunity, and flies.
Child, if you have a rummy kind of name,
Remember to be thankful for the same.
-- Hillaire Belloc, A Moral Alphabet