It's easy to misunderstand this argument because it comes from a worldview and conceptual context alien to the modern discourse. Classically--in the Neoplatonic tradition that Augustine absorbed into the early church--evil is considered a "privation." In other words, light exists, but darkness is just an absence or "privation" of light. Heat exists, but cold is just an absence of heat. Similarly, the existence of good (or Good) demands an explanation, but evil is just an privation of good.
When you conceive the most horrible thing ever, you're really just picturing a situation where almost all good is absent. The two situations (conceiving the good, conceiving the evil) are therefore not analogous or comparable. The argument makes sense solely in the context of the idea that Realness and Goodness are the same thing, and thus that the upper bound of any Real quality is God, the lower bound is non-existence. The Great and the Good and the Real necessarily coincide in this conception (they are three aspects of the same thing)--the "horrible" cannot be substituted in. (Platonically speaking, becoming more Real would necessarily make the horrible thing LESS horrible. The maximal horrible thing has minimal Real existence.)
It's worth noting that another early non-Christian source of Christian theological concepts is Zoroastrianism, in which good and evil are opposing forces of equal reality and efficacy. This concept had a strong influence on the common Christian folk theology of God and the devil as battling it out over the world, but was considered a heresy by the official church. We have to assume that Anselm is looking at this Platonically, and not as a materialist, or as a Zoroastrian, not just because of the position of the church, because otherwise the argument doesn't make even minimal sense.