Not a cactus, but you're very right is a Euphorbia. I don't think it's E. enopla. Thats a thinner plant. Maybe Euphorbia pentagona:). This is were it gets confusing because I see pentagona and enopla get sometimes confused as the same plant, but enopla I have and is thinner. From what I've read pentagona has 5-6 ribs. Where enopla has 6-7 ribs. From what I can see yours has about five. So I'm going with E. pentagona;). There is Euphorbia ferox, but I have that one and it's more rotund;). Oh ya I have to let you know because I would have a guilty conscience if I didn't. When cut or bump this plant will ooze a white fluid. Reminds me of watered down Elmer's glue. This is toxic so keep off skin and away from mucous membranes:). People have gone to the ER because of this stuff. Where your PPE. There you have been warned;).

Thank you. Do you know what kind of mammillaria?

Just plant it into a fast draining mix like 60% rinsed strained perlite or pumice and 40% cactus soil. No water till roots form, which should take about a month. Could take longer so don’t be alarmed if it does. Also since there are no roots keep in bright filtered light. Once roots form you can water and acclimate to stronger sun. It’s some sort of an Opuntia. Since you found it at Lowes I would not be surprised if it was a hybrid. There’s this one I see a lot called Opuntia quitensis ‘Johnson’s hybrid’ that looks similar to yours:).

Opuntia ficus-indica It will need strong sun to grow nice pads:). Definitely an outside plant if you can.