Anyone remember malenti? They are sahuagen that look like sea elves and spontaniously occur (ie some eggs hatch into malenti) when sea elves are within so many miles of a sahuagen community (Sea Devils gives numbers). Using that idea, 1% of unborn humans are changed to half-orcs when true orcs are within 50 miles of a human community. And the same is true when humans are within 50 miles of an orc community. The birth is normal, though the mother may not be emotionally stable thereafter. Survival rates for the half-orcs are about the same as some are raised and most are slain. Half-orcs themselves are sterile.
What other monsters could use this? Or monsters that can only breed by this method?
What other monsters could use this? Or monsters that can only breed by this method?