http://en.ff14angler.com/fish/...
I think how it works is this: You catch 3 Little Thalos to trigger "Fisher's Intuition", a temporary buff. While you have that buff going, you'll have a chance at reeling in the Nepto Dragon.
(Having the Nepto Dragons in your inventory doesn't matter, you just need to catch 3 of them to get the temporary buff going)