The sheer size of Wailord presents many challenges for trainers even today, for they have poor mobility on land, and even in water rarely find a battlefield large enough for them to maneuver. Indeed, many trainers favor opening matches by tossing their poke balls high into the air and having their Wailord land on their opponent with a Heavy Slam.
The very fact that these trainers can throw poke balls is proof of how far we have come, for Wailord were the last intelligent creature on earth (excepting man) to become a pocket monster. The first poke balls were only capable of holding pokemon the size of Pidgey or Rattata, but a continuous series of inventions shrunk larger and larger pokemon until even Steelix and Tyranitar could be caught, placed in small balls and made to obey their trainers. But the funding for and interest in research soon dried up, for only one pokemon was left untamed, so Wailmer trainers were left saving up for Everstones or parking their pokemon at docks as though they were boats and leaving them off their team when on land.
The Heavy Ball which tamed Wailord ultimately came from a large association of deep-sea fishermen, who saw immense profit in Wailord meat and oil and sought a way to prevent these pokemon from accidentally eating their rides (boats and water pokemon alike,) in accidents made possible by their enormousness. But although Wailord could be captured, they are far too big and powerful to be killed like Magikarp; harpoons do little against Water Spout. Wailmer trainers, once the very activists who protested this research out of fear that it would devastate the Wailord population, have become the primary customers for the Heavy Ball these fishermen developed.