Which desert animal stores fat in its hump?

The camel, and the hump is fat rather than water, which is the correction everyone half-remembers. It is a food store concentrated in one place instead of spread under the skin, which matters in a desert: an insulating layer all over would stop the animal shedding heat, so the fat is piled somewhere it can be carried without cooking its owner.

The water tolerance is real but works differently. A camel can lose a quarter of its body weight in water without serious harm, where about a tenth would kill most mammals, and can then drink over a hundred litres in a few minutes. Its red blood cells are oval rather than round, which lets them flow while the blood is thick with dehydration and survive the sudden flood afterwards.

It also simply refuses to sweat until it has to. A camel lets its body temperature drift by six or seven degrees through the day, storing heat rather than spending water to shed it, and dumps the excess into the cold desert night. The nostrils reclaim moisture from each breath, and the kidneys concentrate urine to a syrup. Nothing about the animal is accidental.