pretty sure you have to beat them into submission, and there is no den den mushi on any island; or there is but they can only be used with science and dont count as creatures (currently anyway)
I know that, it'll probably be a while until I have Beast Mastery in canon anyway. This is mostly just for a special BG for now, what if it's not necessarily a Den Den Mushi but just a normal snail lol
This is pretty much my plan for it;
Baby Den Den (or Snail) - Level 94
×2=188
Strength: 1
Vitality: 87+15=102 (100 Stamina/104 Durability)
Speed: 1
DF (Heal): 100