Ugly Fish was added in Beta 9.2. Previous Versions can be found here.
Whenever Ugly Fish damages an enemy monster, that monster will gain -2 ATK, no matter if Ugly Fish attacks or counter-attacks. This is a very powerful ability, able to cripple down a couple of monsters while it lives due to its high HP. The effect also works when it ends up dying in the process, as long as it makes contact with another monster.
As such, the best counters are by killing or simply removing it with spells and effects. Muffet's Pet in particular is useful due to its 2 ATK.
Speaking of Pet, Ugly Fish can work very well alongside it, due to its effect being used to lower enemy monsters' ATK stat to be in Pet's killing range. This works even greater by giving Ugly Fish Taunt by using Protection or Glad Dummy.