The term furniture (家具 kagu?) is used to describe a variety of servants created by those with magical abilities.
The term is used to refer to any beings who are less than human and was most likely created by Yasu. The descriptive term is rooted in Yasu's idea of being a lesser being to ordinary humans, equivalent to the Furniture people use to help themselves. This is probably the reason that all forms of Furniture are servants. However, in Turn of the Golden Witch, Jessica Ushiromiya surmises that it is derived from the phrase "servants should be living furniture."
In Requiem of the Golden Witch, it is suggested that from a real world perspective, Furnitures were drawn by Yasu in some sort of drawing book (which could be Maria's grimoire), which explains how a drawing of Sakutarou was found in Maria's grimore during Alliance of the Golden Witch.