Hello and welcome to FF Sarah ! Without a necropsy it's impossible to tell what caused death . You state left mid-June until October; did person caring for frog knew what he/she was doing and maintained proper parameters for frog enclosure? Was a feeding log kept? A 2 gallon critter keeper sounds a bit small for a 1.5 year old frog. What was length from snout to vent? You might want to read this before getting a new frog: Frog Forum - Pacman and Horned Frogs - Ceratophrys - Care and Breeding. Good luck!