Zombie Creepers are a slow-moving, slightly uncommon hybrid that are rarely spawned when a creeper of any type is killed (12% chance). They walk 2x slower than a normal creeper and will attack anything that aren't undead mobs. They were born when a Zombie and a Creeper were bred together, or are sometimes plainly viewed as creepers that are dead and not nessesarily hybrids.
Zombie Creepers will simply follow anything that attacks them slowly but surely and will ram themselves into their target forcefully likea zombie, but when in a 1-block radius they will try to explode. You have three seconds to run away, as the Zombie Creeper will deactivate its timer and continue to chase you. They will drop 0-2 Rotten Gunpowder when killed.
Zombie Creepers have a 41% chance of spawning anywhere in a 6 x 6 area and in duos. Another way to find one is after killing a creeper (of any kind), there will be a 12% chance the creeper will get back up and begin chasing you.