Does the Hadith saying:
For every flesh that is grown up from unlawful earnings, Hellfire is most entitled to it
apply to a foundling?
The Hadith which is mentioned in the question generally applies to eating from unlawful sources. It is a threatening Hadith that does not by any means include a foundling. This is because a foundling did not commit any sin and no one (Part No. 16; Page No. 9) bears another's burden. Allah (Exalted be He) says,
...and no bearer of burdens shall bear the burden of another.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.