According to Out to Africa, enemies of hippos are lions, crocodiles and humans. However, it is unusual for lions, even large prides, to take on a hippo.
Although lions have attacked hippos before, as evidenced by National Geographic, it doesn't happen often because it puts lions at risk for significant injury. Humans are another danger to hippos. People hunt them largely for their ivory teeth and hides.
One of the reasons hippos have so few enemies is because of their naturally thick hides. Their skin can weigh up to 1,000 pounds alone and is especially thick on the vulnerable back and rump areas.
Hippos never sweat. Instead, they loll about in water or mud to remain cool. There is a myth that hippos sweat blood, which is false. However, it is true that they excrete a red fluid from a specialized gland during times of excitement.
Hippos are found primarily in Eastern Africa around water. They have a sedentary lifestyle requiring very little food. They subsist on eating grass. While they spend much of their time relaxing in water, they are aggressive animals that fight on an almost daily basis. Despite this, they are sociable animals, existing in groups of up to 15 hippos.