Living Fungi

Living Fungi are Overworld creatures that spawn in the Mega Taiga. Living Fungi naturally spawn as a small mushroom (1x1 block) on the ground. When the Mushroom is hit, the Living Fungi will transform out of the mushroom form and into its creature form. While in the ground, Living Fungi will not move nor attack players.

While in the creature form, Living Fungi are extremely dangerous. Living Fungi have multiple abilities that make them a hard creature to fight. Whenever a Living Fungi hits a player, multiple events occur. The Living Fungi will heal 30 health whenever it hits a creature. This healing effect is doubled when hitting a creature that is below 50% health. Upon being hit by Living Fungi, a strong poison effect and strong slowing effect occur. Each of these effects last for 3 seconds.

Living Fungi move fairly quickly and are immune to knockback. They should only be fought with excellent equipment that will easily deal with the abilities they possess. The Living Fungi is a hunter mob that can only be damaged when the player is level 77 or higher. It has the Attributes of Gun and Bow Immunity.

Warning: Do not bring pets anywhere near these mobs because they will aggravate a large group of them and you will most likely lose your pet(s) life.

References/Sources
http://adventofascension.gamepedia.com/Living_Fungi