The park is home to more than 30 different mammals, including the Royal Bengal tiger, leopards, langur, rhesus monkeys, civets, hyenas, wild dogs, sloth bears, otters, blue bulls, deer & wild boars. It is a regenerated habitat for endangered animals like the tiger, rhinoceros, wild elephant (the park harbours the biggest wild tusker in Asia) & black buck, Gharial & marsh mugger crocodiles the great game fish-masher & the Gangetic dolphin cavort in the river waters. The unique birds which number over 350 species include cormorants, egret, storks, & endangered species such as the Bengal florican & the saurus crane thus making the park truly a bird watchers paradise.

The park is home to more than 450 endangered Asian one-horned rhinoceros and harbours on of the largest population of the elusive & rare Royal Bengal tiger. Besides rhino and tiger there are four species of deer, leopards, langurs, sloth bears, wild boars, rhesus monkeys, wild dogs, white stocking gaurs and many other smaller animals. Gharial and Marsh magar crocodiles cavort in the river water. The unique birds which number over 450 species including cormorants, egrets, stork, geese, jungle fowl, king fishers and endangered species such as Bengal tiger florician, Lesser florocian, Brahmin, duck, pintails, Indian pitta & parakeets.

The Reserve is an example of one of the finest birding sites in Asia. Koshi Tappu was established as a wildlife reserve in 1976 with an aim to protect the last remaining population of Asiatic Wild Buffaloes in Nepal. The reserve has a total area of 175 sq. km. and is roughly rectangular in shape. The principal habitats in the reserve include: wetlands, grasslands and small patches of riverine forest. To date this is the only Ramsar Site (a wetland of international significance) in Nepal. More than half of Nepal's birds are recorded from Koshi Tappu! Among the mammals, besides Wild Water Buffaloes, the elusive Gangetic Dolphin and Fishing Cat are occasionally seen here.