Sorry, but it definitely is not a Senita. I can see why you might think that, but if you look in detail at the ribs and overall appearance of the two cacti you will see that the plant in the OP here is not a Senita.
Sy's ID is probably pretty close to what it is - a straight up Cereus of some sort - these occur is South America, can get really big. Will have large mostly white flowers that open at night and will stay open into the early morning. If fruit forms it will end up looking somewhat like an apple, hence the common name.
The Senita is a cactus that occurs mostly in Mexico and in southern Arizona, it also can get big multi branched, but not quite as massive as most Cerei. It is also a night bloomer, but gets much smaller pink-salmon colored flowers, that in my experience are usually closed up before dawn.
If I have time tonight I will try and post some pictures.
