There are a few mammals that cannot jump. (Jumping is defined as getting all of the body off the ground at the same time, "to spring free from the ground or other base by the muscular action of feet and legs." MW)
Fred, I see that site as contradicting itself. Even though I would love to have an independent source agreeing with me, I have to point out that the site says that hippos can't jump, but also says that hippos can run. Running is merely controlled jumping, in my opinion*; all 4 legs (of a quadruped) must be off the ground to be consider running. The only way that site can be right in both cases is with a very loose (read innaccurate) definition of "run." Many sources will state that an elephant "runs" at X mph, yet elephants can't run at all. But they can walk about as fast as the best 100m sprinters.
* MW - run - 1 a : to go faster than a walk; specifically : to go steadily by springing steps so that both feet leave the ground for an instant in each step
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