"The horse has vegetables."
Translation:Tá glasraí ag an gcapall.
For better or worse, Irish has dialects. The course is designed to teach "standard Irish", but a decision was made to be welcoming to those who had already learned Irish from one or other of the dialects, and to accept those variations as correct answers.
Unfortunately, Duolingo doesn't have any direct support for dialects, so in cases where you have "Mark All Correct" type questions, Duolingo expects learners to select the dialect forms that are correct in different dialects.
The simplest solution would be for Duolingo to do away with "Mark All Correct" type questions.
After certain simple prepositions (ag, ar, as, chuig, faoi, leis, ó, roimh, thar, trí and um) and the singular definite article an, you eclipse the following noun. ag an gcapall, ar an mballa, leis an bhfear, etc.
In Ulster Irish, you lenite rather than eclipsing in this case.