Sealock Monster is a uncommon Creature — Octopus card from Theros (THS), collector number 62. Current market price: $0.11 (foil $0.41), TCGplayer market prices via Scryfall.
This creature can't attack unless defending player controls an Island. {5}{U}{U}: Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.) When this creature becomes monstrous, target land becomes an Island in addition to its other types.