The species that built the Ringworld in Larry Niven's novel is the Pak. More specifically, the enormous ring was constructed by Pak Protectors, the highly intelligent adult stage of the Pak life cycle. The Ringworld was later populated by Pak Breeders and other hominid species, many of which eventually forgot that their world was an artificial structure.
Ringworld, published in 1970, sends Louis Wu, Nessus, Speaker-to-Animals, and Teela Brown to investigate the gigantic habitat. The structure circles a sunlike star and provides artificial gravity through rotation. Its inner surface is roughly a million miles wide and has an area vastly greater than Earth's, making it one of science fiction's classic megastructures.
The novel itself initially leaves the builders mysterious, which can make other choices seem plausible. The Puppeteers organize the expedition, the Kzinti are represented by Speaker-to-Animals, and the Outsiders are technologically advanced aliens, but none built the ring. The builders' identity is revealed more explicitly in the sequel The Ringworld Engineers, where the explorers also discover that the Pak Protectors have long since disappeared.