A doodle, often drawn by Hank Green, in the shape of a simple angler fish. "Hanklerfish" is a portmanteau of "Hank" and "anglerfish." (Note: A Hanklerfish doesn't have to be drawn by Hank Green. Any anglerfish drawn by any person named Hank is a Hanklerfish.) 

The Hanklerfish is commonly drawn by Hank Green as a form of signature. While John Green has the "J Scribble," Hank uses the Hanklerfish. The Hanklerfish has even been made into a Hanklerfish Plushie for sale on the DFTBA website.

Background: Hank wrote and performed an original song officially called The Anglerfish Song Regarding Human Mentality In Respect To How To Interpret Perpetually Felt Emotions While Educating You On The Subject Of Anglerfish. However the song is labelled on the song list for the album This Machine Pwns n00bs as  "A Song About an Anglerfish" and the song first appears in the similarly named Vlogbrothers video uploaded February 11, 2009. Hank contributed to John's feat of signing the entire first printing of The Fault in Our Stars by signing some of the books with his Hanklerfish. The popularity of this rare signature spawned another phrase, Hanklerfishing, in which nerdfighters searched for the elusive Hanklerfish inscribed in copies of TFiOS.

The concept has also spawned a Tumblr webcomic titled Hanklerfish comic. The comic is based off of this inside joke, and features anglerfishes drawn with the face and complexion of the vlogbrothers. It updates regularly with no official schedule, and is drawn by Sylvia. The comic can be found here.