Euphorbia horrida hibrid
(This plant comes in a 3.5" pot. You will receive a very similar plant to the one in the picture. It may or may not be blooming at the time of your purchase. The plant is shipped in its pot to prevent any damage to the roots).
Euphorbia horrida is a spiny succulent shrub with irregular basally clumping stems. In age it will forms tight clumps of upright columns up to a meter wide that closely resemble some cactus species. It also has clustered crowns of somewhat ferocious spines much like the true cacti.
Remarks: This plant is somewhat variable with many varieties and forms, and perhaps natural hybrids between it and taller Euphorbias.
Some of the information in this description has been found at desert-tropicals.com, llifle.com and cactus-art.biz