Where does “Jeleń” come from?

Jeleń (Polish) comes from Proto-Slavic (j)elenь, from Proto-Indo-European h₁el-en-i-.

Ancestry of “Jeleń”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	(j)elenь	—
2	Proto-Indo-European	h₁el-en-i-	—