Some think the Hattifatteners are wicked, but this is due to their strangeness.
Hattifatteners are found only in groups, and often in swarms. Although they do not possess a hive mind, they agree so thoroughly on the fundamentals of Hattifattener logic that a player may play any size group of them as a single entity. They excel at problems of pure logic and electrical engineering. For Ars Tartary it is sufficient to state that they have no skill at combat but they may strike with a lightning-like attack, for 1d10 per turn spent building up charge, up to as many dice as there are Hattifatteners present. D&D variants will want stuff like their HD and special abilities that unlock as you go up levels: these involve the strength of their lightning attack, their ability to bypass simple obstacles by slipping through narrow gaps and/or quantum tunneling, and at high levels a telepathic mind-link power that extracts information and leaves the target confused and strangely affectless. You can certainly figure out the details there. They save and level as Monks.
The number of Hattifatteners present is indeterminate until it becomes germane, at which juncture roll 1d6 + 2, exploding. This roll may be modified by environmental factors – their numbers are suppressed underground, raised when near big electrical charges. Individual hattifatteners are extremely good at dodging but groups cannot react so quickly – they therefore attack, defend and take damage collectively.
Details of the workings of the Hattifattener “barometer” will not be revealed at this time.