One horn rhino comes under Indian rhinoceros clan. You could spot them at
Kaziranga park
Jaldapara park at WB
Gorumara park at WB
Dudhwa park at UP
Dibru saikhowa park at Assam.
Kaziranga, Pobitora (Assam) and Joldapara (West Bengal) are the best places to see one-horned rhino. From my personal experience, Kaziranga is more famous and hence more people go there and as such it is crowded. Pobitora is not so crowded and you can enjoy watching the rhinos in peace. Also, in Pobitora, the density of one-horned rhinos is more.