I finally caught the lil guy after trying a few different methods.

I used the advice of previous discussion posts and focused on building near the water but I took it a step further and wanted to share my tips. It only took a few minutes on a new map with the following setup to get a few (and I still see it's at 0.1% of players).

By day 6 I already had 3 tritons appear (missed the first two bc i blinked).

• Choose a small map with omnipotent and unlimited powers. Turn off migrants. Select Attainment. With this setup you cannot win, but neither can you lose.
  
• Build close to the water so your minions have to circuit around the shore. Use tiles to surround the whole lake with a path. Make sure to set watcher eyes all around the water, and be careful not to build over them.
  
• Use your unlimited crypts to get many skellies marching around the shore. (I have seen it try to abduct demons and skels).
  
• Kill everyone and everything. Kill all the sheep, ruin the town after. Use spells like wind blast to clear everything that is moving. Some spawners may regenerate animals but not for long with you in charge.
  
• Place snare traps all along the coast, at least a few squares deep.
  
• Play on normal speed and prepare to zap if a snare did not get him. They can only be snatched but they should be no challenge. Make sure you have a kennel.

I tried some other things, like using kennels to spawn sheep and boar, or flying creatures like harpy, but I never saw a Triton go for anything but skellies or demons (in my limited focus).

It is my speculation that having no other process, events, or characters to deal with, the 'Triton event' gets rolled more often in the background. For a normal playthrough you will need such quick reflexes that actively searching for one may not be worth your time.

Hope this was helpful for you!!