Llanowar Sentinel is a common Creature — Elf card from Tenth Edition (10E), collector number 275. Current market price: $0.58 (foil $0.50), TCGplayer market prices via Scryfall.
When this creature enters, you may pay {1}{G}. If you do, search your library for a card named Llanowar Sentinel, put that card onto the battlefield, then shuffle.