It looks like a barrel cactus with an unusual top, judging by its shape and spines Sometimes the "growth end" of a cactus, for unknown reasons, starts growing differently than the rest of the cactus. Sometimes the growth is attractive and, by taking cuttings and grafting them to an appropriate plant stock, one can propagate these unusual growths. Several are available on the web. To see some other examples (some will not be as notable as yours), try googling Cereus peruvianus monstroso. A Cereus peruvianus is a kind of cactus. To distinguish the regular Cereus peruvianus from the one with the unusual growth, "monstroso" is added to the end of the scientific name. That's all I got for now. Ben

This is a (believe it or not) brain cactus. They're found in the Sonora Desert in the Southwest. Check it out on www.desert-tropicals.com/Plants/Cactaceae/Stenocactus_multicostatus.html