Indricotherium

Indricotherium stands about five meters tall. An average human, at about two meters in height, is shorter than one leg of this giant 20-ton mammal.
AMNH/D. Finnin

The largest living land mammal is the African elephant. Indricotherium--a herbivore that lived in the forests of central Asia between 34 and 23 million years ago, weighed as much as three or four adult African elephants. 

Indricotherium fossils--found by Roy Chapman Andrews, a young scientist from the American Museum of Natural History, in Mongolia in 1922--indicate this giant was the largest land mammal ever discovered.
 
Weighing as much as 20 tons as an adult, Indricotherium could stretch its long neck to nibble leaves high in the treetops of the central Asian forests. Needing to eat massive amounts of vegetation to survive, Indricotherium suffered as the central Asian forests were replaced by grassland habitats, causing this huge mammal to become extinct.