Sugar lumps were items included in the mod from Public Beta 1 through to Public Beta 6 (inclusive). They were craftable from one sugar item, and could be fed to the old mod horses while riding them to give the horse a temporary speed boost. They were removed in Public Beta 7 as the mod updated to Minecraft 1.6, replacing its old horses with the new vanilla ones, and the new horses had different mechanics meaning that the sugar lump was no longer useful or necessary. This is similar to the mod's original hay bales.