The Bunny (or Bunny-Rabbit/Rabbit) is a rabbit that drops hearts on death, which heal the player. they are one of the only ways to regain health
Bunnies will spawn in multiple locations around the forest, the Canyon, and the Tundra. They will stand around and appear to eat grass. When hit with any kind of attack, they will squeal and disappear, with a red Heart taking it's place. Touching the heart will consume it and restore the player's health to full.
Other ways of healing
Multiple perks exist in-game that provide ways of regaining health other than bunnies, as well as influencing health in some way.
Bloodthirst - Beserker
Bloodthirst gives 25 for every time you kill an enemy player or NPC with an axe throw.
Meditate - Warrior
After staying still and blocking for a short time, you will begin to slowly regain health.
Sanguine Strike - Grim Beserker
Throwing your axe will create a dome that drains health from all enemy players and slowly heals you for that amount. The more players inside the dome, the more you heal.
Ruthless - Warrior
However, when hit with a player with this perk enabled, you will display a "Bleeding" Icon and will take reduced healing, meaning that 2 bunnies are required to fully heal you, instead of one.
Prey - Hunter
Nearby bunnies will be marked with an icon and hearts will bring the HP up to 140 when taken.