Muffet's Pet was added in Beta 2.4 update. Previous Versions can be found here.

When placed, this monster will kill any monster with 2 or less ATK, this means it can kill enemies such as Heats Flamesman, Sans, Monster Kid, Goner Kid and even Mettaton but also cards that have high HP such as Ice, and those who mostly have no attack on their own, but have an ability that activates each turn or each attack, such as Memorial Statue, Cactus, Blue Laser, Orange Laser, Pyrope and Gaster. Since Beta 10.2, Muffet also gets a +2/+1 boost from it, increasing its worth even more. This card is incredibly valuable and it is suggested one should have this in their decks much like the Nacarat Jester. It is also suggested you craft it if you can't get one in packs as it is only 100 dust.