The Ancient Krug is a large monster found on the Summoner's Rift along with a smaller Krug. The Ancient Krug spawns two Krugs when killed, which will each spawn two Mini Krugs when killed.
If the Ancient Krug is killed and the Krug(s) or Mini Krug(s) are spared, they despawn after a few seconds (e.g. Summoner 1 is counter-jungling and kills the Ancient Krug. The Ancient Krug splits into the two Krugs. The summoner kills the two Krugs which leaves behind four Mini Krugs. If the Summoner then retreats, the four Mini Krugs will despawn)