5e doesn't have "Magic Fang", it replaces it with "natural weapons count as magical" class abilities for moon druids, monks etc.
I don't think Magic Fang was in 2nd edition, but I never owned all the books. In 1st edition there was a rule that natural weapons for creatures over certain hit dice counted as magical.
The role of pets was rather expanded in 3rd edition, in comparison to both 2nd and 5th, so it was necessary to give them some way of hitting mundane-weapon immune monsters. It was already well established that Magic Weapon (and similar spells like Bless Weapon) did not work on natural weapons.
Technically "Magic Fang" didn't work on natural weapons either - it targets the creature, not it's claws/teeth directly.
