Originally posted by Veseljko:

Serpents you find during the night or during the storm will despawn. The ones you find on a sunny day (this is rare) will not despawn.

You can't find them in the daytime except during a storm. Only those found during the storm will not disappear. Those found at night will disappear during the day, unless there was a storm at night, but then you are unable to say whether it is born of the night or of the storm so you'd better kill him. The one you found during the day was the one who appeared in the storm. 5% chance for night spawn 5% for storm spawn if both 10%.