Euphorbia enopla (Euphorbiaceae)
Cactus for indoors? Certainly and this would be a good candidate as it only grows to about 1 metre. This is frost hardy species which originates from South Africa has very attractive long reddish spines which turn to black as they age. When I look at this specimen I can see that the side branches could be removed and used as propagation material to grow new plants thus changing the shape to a more central column.