Plural of nyehatt, "his animal". Possessed form of 'ehatt, "dog, horse, pet, domesticated animal", which does not have a plural on its own.
Nyechehaatt in Mesa Grande and nyechhaatt in Tiipay; Cuevas does not pronounce it with the long aa that would be expected as a plural of nyehatt.
Following a similar pattern are nyechehaal "their boss" and nyechewaa "their house". Notice that in all these words the owners are plural.