Bearly Legal is a gameplay achievement for Red Dead Redemption.
The official description of the achievement is: "Kill and skin 18 grizzly bears."
These Grizzly Bears will be encounter during Marston's travels throughout the wilderness and is part of the hunting activity.
The name of the achievement is likely a pun in reference to the term 'barely legal', an axiom with wide use in pornography to convey legally-accepted youthfulness of performers.