Hoomhoffers are large arachnids used as heavy cavalry, close air support or shock troops by Baron Klaus Wulfenbach and later Martellus von Blitzengaard. Each "hoom" (as they are sometimes called) is ridden by a mahout who directs its actions. Normally hoomhoffers
They are constructs reportedly made from enlarged dust mites (so they would be big enough to study), with air pumps installed on their backs that allow an arachnid of that size to breathe. The pumps are an inconvenience, but the hoomhoffer's incredible toughness, strength, newly acquired ability to fly, and fearsome appearance more than make up for this.
Hoomhoffer is also a card —four copies— in The Works; they are also Giant and Bug. However, the depiction is consistent with the claim that this is an arachnid: eight legs are clearly present, including two raptorial forelegs.