Cactus is the New World answer to Euphorbia in the Old World. They formed along lines called "co-evolution"--similar desert environment, similar botanical response in different plant families. Cacti have clear liquid and are considered an off-shoot of the rose family. Euphorbs are noted for their milky juice.

There are five giant cactus in the Americas, and this one resembles a couple, one called Organ Pipe, another called Senita. It could even be a small Saguaro. But I doubt that's what they are, because those do grow very large. Someone might sell them figuring they'd never make it to full size, but there are a lot of intermediate-sized cacti out there also and I think that's what you have. Something smaller that grows faster.

If you poke it with the point of a sharp knife through the skin into the flesh and it oozes a drop of sticky white milky juice, then you have an Old World Euphorb. If it has a clear juice, it's a cactus.

Let's go from there.

SRS