Jagna

The name is actually a medieval vernacular form of Agnes; not necessarily a diminutive. It comes directly from the Polish Jagnię, it was later synchronized with the Latin based form of Agnieszka. It recently became popular in Poland again.

Allegedly a diminutive of both Agnieszka and Jadwiga as well as Agata:
https://pl.wikipedia.org/wiki/Jagna
https://pl.wiktionary.org/wiki/Jagna [noted -ed]