They are about the same size as a Human, covered in chitinous bluish skin and possess Human-like eyes and mouth but also a pair of insectile antennae and one of what appear to be external mandibles or pedipalps. Curiously, though, they are also known to produce milk and feed upon it, a trait more commonly associated with mammalianoid races.
The Malmooth society built huge organic-looking underground cities known as conglomerates, on Malcassairo, similar to gigantic termite nests. A hundred trillion years in the future, they are all extinct except for a single surviving member, Chantho, and their abandoned conglomerates have been overtaken by the descendants of humankind.
Social norm dictates that Malmooth speak each statement sandwiched between the two halves of their own names, for instance, in Chantho's case:
"Chan - statement here - Tho".
They consider it extremely rude to speak in any other fashion, although they don't seem to have a problem with aliens' not abiding to the same habit.
With Chantho's death at the hands of the Master, the species presumably became extinct.