Ice Wyrm to Sand Wyrm

Dungeon Fantasy Monsters 1 features the Ice Wyrm on page 20. It's a giant cold-breathing worm that's especially dangerous if you encounter it on ice. This is very nice if you have the players in a cold location. If you're in a desert, it's less useful... unless you turn it into the notorious and ever-popular sand wyrm!

This is not a terribly hard conversion. 

Remove slippery, cold resistance, and Temperature Tolerance, Detect (Heat), and DR vs Cold

Change the name of the breath to "Sand Blast", and change the effect from paralyzed to blinded.

Change the fatigue damage type on the sting to "Thirst" 

Add vibration sense (air). This works from beneath the sand.

Change the specialty of the terrain adaptation to (Sand)

Add Tunneling 4 (only in sand)

Ice generating organs cannot be harvested, but mana organs for earth and air can be harvested for $4,000 each

And there we have it. Simple as that. 

Now, you may not need a Sand Wyrm specifically, but the general principle applies. Take a monster close to what you want and change it to fit your scenario. If you need tons and tons of elder things, edit existing mundane entries, adding tentacles or removing their eyes (or adding far too many eyes) and alter their fluff a little. That's why it's called "fluff", after all. 

Maybe this is an obvious thing. But I find a lot of GMing is just about fundamentals. So here is a Sand Wyrm, and here is a tool. I hope you find the monster useful! And if not, I hope you find the technique useful!