There Once Was a Dragon Named Meatlug is a short song that Fishlegs sings to Meatlug in the episode "Gem of a Different Color" while they're relaxing on an island that will soon be identified as Changewing Island.
Before he can finish singing, Meatlug gets distracted and walks away, leading Fishlegs to admit the song was bad, even for him.
There once was a Dragon named Meatlug
She was cuter and smarter than a sea slug
When she met her pal Fish
It fulfilled his greatest wish
To love her and give her a big bear hug.