Del Lagos is a boss in Resident Evil 4. After getting attacked by the villagers, the two policemen that Leon traveled with dissappear. The first is found burning at the stake in the village, the second was seen being dumped into a lake by villagers, and is eaten by the giant lake creature known as Del Lagos.
 How to fight
Leon hops aboard a nearby boat, and floats out to the middle of the lake. It is here that the battle begins when Leon's boat gets attached to Del Lagos.
Leon is pulled around the lake, and must first avoid the large floating objects. Leon must throw the harpoons on the boat into Del Lagos's mouth when it turns around. You can also be knocked off the boat and will be forced to mash some buttons so you can swim back to your boat safely.
Related Threadsweird 'Del Lago' glitch - last post by @ Feb 23, 2009
del lago - last post by @ Dec 22, 2005
Saddler: easier than Del Lago? - last post by @ Feb 10, 2007