You can only obtain this from the Lava Dragon,
|Forest||Rabbit · Wolf · Spider · Boar · Hawk|
|Winter||Penguin · Arctic Fox · Polar Bear · Baby Mammoth · Mammoth · Baby Dragon · Dragon|
|Ocean||GMO Piranha · Kraken|
|Lava Biome||Lava Monster · Baby Lava Dragon · Lava Dragon|
|Desert||Vulture · Sandworm|
|Other||Zombie · Vampire · Crab · King Crab|
Warning: This does not have the power of the Dragon Heart, as that heart allows you to be resurrected. The lava heart does not have the resurrection power.
The Lava Heart Item was added in "It smells like burning! (Part 2)" update. Ironically, it had no use until the "Time is Running Out!" update, 5 days later.