lost sheep
Literary or classical term for a sheep that has gone astray; often used metaphorically for a person who has strayed from the right path. Rare in everyday conversation.
There is a classical idiom 'lament over the lost sheep'.
彼は若い頃の放蕩を振り返り、亡羊の思いに駆られた。
Looking back on his youthful dissipation, he was seized by the feeling of a lost sheep.
locking the barn door after the horse has bolted; too little, too late
too many options making selection difficult; truth being hard to find as paths to it proliferate
From Chinese 亡羊 (wángyáng), literally 'lost sheep'. The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.