Geometer's Arthropod is a rare Creature — Fractal Crab card from Secrets of Strixhaven Promos (PSOS), collector number 191p. Current market price: $0.38 (foil $1.08), TCGplayer market prices via Scryfall.
Whenever you cast a spell with {X} in its mana cost, look at the top X cards of your library. Put one of them into your hand and the rest on the bottom of your library in a random order.