Just for fun I timed the spawn and it is basically what everyone has stated at 16min 15secs. No big, but still the first pop of arriving at the camp had the ancient croc spawn and dropping the sleeves.. Very handy indeed.

The Ancient Croc is probably second only to the Shin Lord in terms of popularity. The Ancient Crocodile can be found in the very back of Guk, near the two watery enterances to Lower Guk.
He hits for 72 Max, but has relatively low HP for something which hits that hard.