One of the really interesting things is the rarity of mythics is the same as the old rare cards.It is but mythics exploded the price to play. Their was a deck called mythic and cards like Tarmogoyf and and Jace the Mind Sculptor.
In the original set, there was a Mox Jet every 121 boosters. There was also a Thoughtlace ever 121 boosters.
In recent sets, there is a specific mythic rare (e.g. Tarmogoyf) every 121 boosters.
"Rare" cards became twice as common once Mythic was introduced.