The Weapon Invention action of the kobold inventor in Tomb of Annihilation references Appindex A in its weapon invention action for pot of rot grubs:
Rot Grub Pot. The kobold throws a clay pot into a 5-foot-square space within 20 feet of it, and it breaks open on impact. A swarm of rot grubs (see appendix A) emerges from the shattered pot and remains a hazard in that square.
However, appendix A only contains the two new backgrounds. Is this a typo, or did I just miss something? Also, if the stat block isn't there, where would I find the stat block?