Where does “э-” come from?

э- (Russian) comes from Proto-Indo-European h₁é, from Proto-Indo-European ǵʰe.

Ancestry of “э-”, step by step

Step	Language	Word	Meaning
1	Proto-Indo-European	h₁é	and, then
2	Proto-Indo-European	ǵʰe	—

э- — Etymology, Origin & Word History | Etymology Explorer