The tail of a hippopotamus is fairly short, measuring about 18 to 20 inches. But verse 17 says that it's the actions of its tail that are compared to a cedar, not that it looked like a cedar. This corresponds well to the hippopotamus because the animal has the ability to set its thick tail rigidly upright or swing it about like a tree.
Animal Creation Magnifies Jehovah (w06 1/15 pp. 13-16; "Behemoth Glorifies God")
BEHEMOTH (Insight-1 pp. 280-281; Watchtower Online Library)
Dinosaurs - When Did They Live? (Search For Bible Truths)