Your search for "Merfolk Sovereign 164" revealed the following:
Merfolk Sovereign (LCC) from The Lost Caverns of Ixalan Commander, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Marta Nael Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature can't be blocked this turn.
Merfolk Sovereign (M10) from Magic 2010, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature is unblockable this turn.
Merfolk Sovereign (Foil) (M10) from Magic 2010, available at 401 Games Canada. Rarity: Rare Finish: Foil Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature is unblockable this turn.
Merfolk Sovereign (M11) from Magic 2011, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature is unblockable this turn.
Merfolk Sovereign (DDT) from Duel Decks: Merfolk vs. Goblins, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk can't be blocked this turn.
Merfolk Sovereign (J22) from Jumpstart 2022, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature is unblockable this turn.
Merfolk Sovereign (Foil) (M11) from Magic 2011, available at 401 Games Canada. Rarity: Rare Finish: Foil Artist: Jesper Ejsing Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature is unblockable this turn.
Merfolk Sovereign (E02) from Explorers of Ixalan, available at 401 Games Canada. Rarity: Rare Finish: Normal Artist: Slawomir Maniak Card Text:Other Merfolk creatures you control get +1/+1.T: Target Merfolk creature can't be blocked this turn.