Translation:Rats are living in the horses' stable.
Yes. This is a very annoying bug that happens with all plural possessive s' in Duolingo. We have reported it to staff and they have told us that they are working on it. If you would like to file a bug report, just take a screenshot next time it happens. Here is the bug form.
Yeah glad to know someone reported the 'horses' situation, it threw me way off