Time Walk Taking extra turns in just about any format can get you a lot
Like most Magic: The Gathering cards, the value of Briarhorn depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (The List)
Each Core Set 2021 Collector Booster Pack contains 2 foil rare/mythics (30% chance of upgrade to foil extended art), 1 non-foil extended art rare/mythic (possible buy-a-box card), 1 non-foil rare/mythic special treatment card (showcase, alt-art/alt-frame, or borderless planeswalker), 1 foil special treatment card (showcase, alt-art/alt-frame, or borderless planeswalker of any rarity), 2 non-foil showcase or alt-art/alt-frame (common or uncommons), 1 foil showcase common or uncommon, 2 foil uncommons, 4 foil commons or lands (basic or dual), 1 foil showcase basic land, and 1 foil double-faced token.
*Full house, three cards of one rank accompanied by two of another, such as 7-7-7-J-J
If you want to practice drafting a set beyond what's available on Arena look though the list of all the sets Draftsim has to offer
Finally, Elvish Regrower gets my vote for best green uncommon