A dianoga is in fact canonically not worm-like at all. It's more like a giant octopus/squid thing. Not that you could tell from the low budget rubber monster effects in the original movie, which really only represented a tentacle and an eye stalk.

You can use this trick in a game, too. Something similar to the Watcher in the Water from The Lord of the Rings. Have whatever the PCs initially fight and think is a minor inconvenience turn out to be just part of a much larger creature which is initially hidden.

This is sometimes done in movies (like Star Wars) for budget reasons, because you don't actually have to show the entire monster. But roleplaying games don't have special effects budgets, so you can unleash the full horror on them when the time is right. Like when they're down to half hit points or less, and think they've almost won the fight.

Transcript

GM: <roll> Luke, you're surprised by the... <scribble> dianoga. It pulls you into the water.
Luke: Whoa!
Han: Luke! Quick, dodge!
[SFX]: Pow! Pow!
GM: You can't see the dianoga to hit it. Luke vanishes under the water.
Han: Wait, it's a big worm, right?
Han: Luke, you're a Jedi, right? Cast Summon Bigger Fish!
Luke: {under the water} Let me check my character sheet here.
C-3PO: You can't kill it! It's a native creature of Naboo. It's the last of its kind!
C-3PO: Artoo, do something!
[SFX]: < doop bzzt boodle pow >
R2-D2: Okay.
R2-D2: Starting the compactor.