Euphorbia horrida tends to be variable in form and appearance. It also tends to hybridize with some other members of its genus. Both of these features complicate identification. The green fruit distinguishes it from E. polygona, which has red or purple cyathia, whilst in other aspects the two species are very similar.
Maybe somebody could have found a nicer name for it? Horrida refers to the spiny aspect of its appearance, not calling the plant horrid! Some of the spines are actually the stalks of the male and female cyathia that grow on separate plants (www.cactus-art.biz).