Shark Puppy Origins

Shark Puppy exists as part of the Stephen King Multiverse, and was first brought to the mainstream attention of the IT fandom by Tumblr user @billdenbrough in a post about the acknowledgments sections of the King books The Girl Who Loved Tom Gordon and Duma Key, though the earliest apparent recognition of these texts was in early 2008, shortly after the publication of Duma Key, on a discussion forum for King called darktower.org.

Textual References
The acknowledgments of The Girl Who Loves Tom Gordon reference permission to use lyrics from a fictional song called "Gotta Get Next To You (Jus' Slip Me A Taste)" written by Richie "Records" Tozier. The referenced lyrics are "Put your arms around me ... cuz I gotta get next to you," sung by Trisha on p. 203.

The acknowledgements of Duma Key reference permission to use lyrics from another fictional song attributed to Richie - "Dig" by Shark Puppy, with writing credits to both R. Tozier and W. Denbrough. The referenced lyrics are "Life is more than love and pleasure, I came to dig for treasure. If you want to play, you gotta pay, you know it's always been that way. We all came digging for treasure," heard on the radio on p. 313 and quoted on the epigraph opening the book.

The copyright dates for both songs are in 1998 and 1986, respectively- the earliest being shortly after the Losers leave Derry for the second time in the IT novel timeline, and the next being significantly after this departure, which is canonically followed by their secondary case of rapid-onset supernatural amnesia, resulting in them not remembering each other for the second time, this time permanently, and thus unable to collaborate. For this reason the song attributions cannot be considered canon without an overarching unifying theory to account for the discrepancy.

Multiverse Explanation
Explanation for these attributions lies in the King multiverse, in which several of King's novels take place within a self-referential system of parallel or alternate universes. The events of IT take place in the mainstream universe, though are affected by the macroverse- the space outside of but containing the remainder of the universes, and where the beings Maturin and It reside.

This understanding lends itself to the knowledge that, as both Maturin and It are extra-universal beings, their presence and effects are not limited to the scope of the mainstream universe, i.e. Derry, Maine and the events of IT. It is implied that the eldritch abomination of Duma Key, Perse, is either a manifestation of or similar creature to It, possible due to the split timeline caused by alternate universes. This is further supported by the appearance of Deadlights and their associated Glamours explicitly referenced in otherwise unassociated works, such as Later.

Therefore it can be extrapolated that, in the Duma Key universe, the Losers are not responsible for fighting It, as It is now present in Florida instead of Maine, leading to the events of Duma Key and allowing for the canonical formation of Shark Puppy uninterrupted by the malevolent influence of It.

Fan Response
@billdenbrough 's post continues to describe headcanons for Shark Puppy as a band AU, detailing instruments, possible songs, and band roles.

Stemming from this first analysis, many other fans have created memes, developed collaborative lore, written fanfiction, and even recorded songs under the united Shark Puppy moniker. Though these works are collaborative in nature given the specificity of their shared topics, they differ due to individual preference, and as such the body of Shark Puppy work is by necessity as a whole all canon-divergent past a certain point. This results in, among other things, multiple band formation headcanons, multiple musical dynamics, and multiple versions of the "same" song.