Fen Hauler is a common Creature — Insect card from Aether Revolt (AER), collector number 58. Current market price: $0.05 (foil $0.10), TCGplayer market prices via Scryfall.
Improvise (Your artifacts can help cast this spell. Each artifact you tap after you're done activating mana abilities pays for {1}.) This creature can't be blocked by artifact creatures.