Maybe a member of genus Echinopsis (AKA Trichocereus), it kind of looks like san pedro (Echinopsis pachanoi) but can't be sure. San Pedro is pretty common as its hard to kill and grows fast (also it contains mescaline, but don't worry, it is legal). Maybe just look up echinopsis plants and see which one fits its description.
heres a link to a pretty complete list of different genuses and species of cactuses.
http://cactiguide.com/cactus/?start=...opsis&species=
