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 areaand 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.