As far as I can figure out they either evolve when exposed to copious amounts of drooling tiger-dogs and hyena-things, or it's just a matter of being extremely patient and training them a lot.

[It's almost like the nebulous forces ruling Johto don't WANT it to be easy to obtain the giant sea-snakes of destruction.]

...and it might warrant some scientific examination, but I'm fairly certain it's not the first option.