I also had the same thing happen to me but with my chicken (Matilda) and the blue egg I brought. They disappeared from all inventories, even after leaving the fair. Matilda was still showing as being registered with her coop but she was not physically anywhere in the game; same with the egg. At the time my sim was pregnant compared to another save file (that I deleted) where my sim wasn't pregnant. So to test my theory that being pregnant confused the fair load; I recovered a previous save before she was pregnant and I was able to bring Matilda and the egg to make a fair appearance.
Don't know why pregnancy did this but it did. Maybe this can be fixed?
