I read this book to my children between 1966 and 1970. The story is about a rhinoceros who escapes from a zoo and two children, a boy and a girl, find the rhinoceros in a park and see to it that he gets home.  My recollection is that the book had a brown cover.  I don’t remember it having a dust jacket, although it may have when it was new.  We must have purchased it at a local bookstore in Northridge, California.