。
Even good-looking horses can weaken at times.
Literal
Good seem-looking horse [even-でも] sometimes weakens things-[subject-が] exists.
~ことがある ('there are times when X') marks occasional occurrence. 弱る ('to weaken, grow weak') is intransitive. 良さそう is the 'looks good' conjectural (~そう attached to adjective stems). A quiet observation about deceptive appearances.