Ideas
I just had two mini goats vanish from the treehouse lot in Everdew. Original house was demolished and I put a Fairy Pod house I downloaded from the gallery. Not sure why that would have anything to do with it though. My sim had had them for about three or four days and they were happy, fed and everything good. She had been going to work and they were still there when she would return. The only thing I had her do different that day was go get her daily allotment of fairy dust. Doing so doesn't require her to zone in or anything since it is in the same vicinity. Then it was time for work so she went. All I can think is maybe the goats tried to follow her when she went to get the dust and when she departed for work from the location outside her house, it caused the goats to glitch since they would not have been on their home lot. However, that is just a theory as I never saw them off the lot. When she got home from work, they were just gone. Not in inventory or anywhere else that I could find.