Echo Fish is a card added in Beta 7.4. It's a rare card, thus can be summoned by Igloo.
This card works somewhat like a very old version of Echo Flower, being able to copy a monster from the opponent's field. However, it won't give a copy into your hand. Instead, the ATK, HP, Charge and Haste of the selected monster will be copied to this card.
This card can be useful both early game, for when the opponent placed cards like Lemon Bread or Tsunderplane, but also mid- to late game, when the opponent places a 10+ G card, such as Glyde, Onion San or most Legendary's/any DT's, which gives this card brutal stats for only a 5 G monster. This is especially the case with Papyrus and Casual Undyne.