Aqua Rex is an enemy in Final Fantasy Dimensions. This enemy makes use of abilities that inflict physical damage to the party. There should be a White Mage or Seer casting Protect and Curaga while other members attacking the enemy if the party cannot finish it in several moves.
The enemy's Japanese name is from the English "Sea Rex". This is likely a pun on "T. Rex" and "sea".
Aqua is Latin for "water".