I mean, maybe they just thought it would be fun for Mareep to have it?Why does Mareep learn Thunderbolt by breeding in Gen II? In other gens it's a TM, fine. But it was semi-exclusive by level-up in that gen, with only the Pikachu and Electabuzz lines learning it naturally. I'd understand if it was widely available by breeding as a way to make up for it not being a TM, given out to Lanturn/Girafarig/etc, but just Mareep feels like an oversight.
I guess you really wanted to give an explanation, the fluffy fur building up to a thunderbolt lent itself to an egg move.
They also gave Psyduck Ice Beam by egg move in Gen 2, so if anything the real oversight was not giving Flamethrower to one (1) Pokemon by egg move.