MTT Fountain was released in the 7.6 update, along with many others. Previous Versions can be found here.

This card is very unique. Whenever it takes damage, the same amount of damage is restored on its owner. So if used right, it can reward you better than with cards like Shyren.

Plus, just because it has 5 HP, does not necessarily mean it can only heal you by 5. For example, if it is at 1 HP, and gets damaged by a monster with 5 ATK, this card will heal you by 5 HP before dying. So when you face one, try to use as little ATK as needed.

Try buffing this card's HP with cards like Force of Nature & Longevity (in Kindness) or Migospel (in general) to maximize its effectiveness. You can also increase its ATK with cards like Strength & Aaron's Secret (in Bravery) or Snowdrake (in general), so it cannot be affected by Muffet's Pet.

And speaking of Muffet's Pet, this card will only activate its ability upon taking damage, so insta-kill cards will not trigger this.

As with all 1/x cards, this card is also very weak to Madjick and Silencers like Nacarat Jester and Scarf Mouse.