Nova Creepers are the rarest and most widely-known species for it's very common (and very dangerous) explosive instincts. They are as tall as Wither Skeletons, but when they detonate, the crater may lead all the way to bedrock.
Nova Creepers are not naturally agressive mobs, but when provoked at least twice, they curl up in a ball and begin to hiss. They detonate in about 20 seconds, and depending on chance, the size of the crater may be up to 100 blocks wide. They can also detonate and leave a crater in water and lava. The initial damage from the explosion could easily kill a player, even in a full set of Diamond armor. The yellow parts on their body glow before curling up, then the Nova Creeper slowly turns white. When killed, they have a 50% chance of dropping 1-4 Nova Gunpowder.
Nova Creepers, in the Overworld, spawn in Volcano Biomes only. Even so, for every 20x20 block radius, there is only a 0.0000021% chance that they will spawn, making them extremely rare. However, the alternative way to find Nova Creepers is in their naitive and sparsely known dimension, Novacios (NO-vah-SEE-ohs), having a surprisingly high spawn rate of 80% for every 5x5 radius compared to their highly unseen numbers in the Overworld.