My 2 cents.
If you want your Nessie to look as much like the "real" Nessie I suggest pieces.
To make it solid or use and armature? For the hump or back you could always make mound of damp sand or dirt and pour the "hump" right on the mound, lay in some chicken wire or fiberglass mesh, and sculpt the hump to your design. When cured remove from the "earth" mold, turn it over and wash out any residual sand or earth, and place it anywhere.
I suggest the neck and head be a hollow armature of formed chicken wire or similar material.
One reason for making the parts hollow is the weight and another I borrow from Sculpture in Concrete by John Mills: "the sculptor usually achieves the tensile strength so important to him, by making freestanding forms hollow, working on the principle that, by its very nature, a hollow sphere has a greater strength than a solid sphere, and a tube has a greater tensile strength than a solid rod."

There was one in a day care near us at one time that you might downsize. They used tires buried (kids could go thru or over). That was all the humps. They used a partial one (maybe) for the tail (cut out part of it and buried it all but about 1/4 of it and had cut the tail out of that. The head was made of plywood I believe and came up about the same height of the middle hump and painted and sealed. Not sure how they got it to stay in the ground. Maybe posts and cement. You could use small wheels.

I'm pondering the exact same thing!! And another dragonfly.
And a snake.

Here is a link that might be useful:

Okay, I know I have seen the tire "Nessies" but this one is really cool, it is a pre-formed wire piece meant as a topiary frame. Pretty steep price but it could be a source of inspiration.

Here is a link that might be useful: Nessie Topiary Dragon

nmgirl, Love your idea to make old Nessie! I would think to make it in sections would be easier to handle; also to move around to a new 'spot' if at any time you so wish, plus if one hump, would happen to crack or break, easier to fix or make a new section. Covering large plastic bowls could work or even covering 1/2 to 3/4 of a plastic beach ball for a hump (like we do for spheres) could give you a nice size. As one of the posters above suggested, using a mound of sand or dirt, make sure it is extra damp (so it don't move) could give you any size you want. If you want more of the 'Nessie' to show I would think a chicken wire armature would work great - once you have it formed (use leather gloves for this). Good luck and be sure to post pics. for us all to see.
luna_llena_feliz, love the link-thanks for sharing. Wendy

Thanks for all the input!
Robert: I like the damp sand pile idea, I use that method for leafcasting. I hadn't even thought about it for Nessie. I also need to find that Sculpture in Concrete book.
BTW, great dragon!
Two Months:
I've seen tire Nessies but that isn't the quite look I'm hoping for. I have a friend who has a tire "earthworms" scattered through her humongous garden, actually it's more like a landscaped pasture. She's given them, the worms, all different personalities. One of them is a red hat worm, another is a mama worm followed by her "kids" which are made of bike tires, she even has one with a beard. Very fun and her grandkids love it!
Love the giant rattlesnake Calamity! Oh dear, another idea....
Luna:
Topiary Nessie? That hadn't occurred to me. I don't know that I have the patience to wait for the plants to fill it in but I do like the idea.

Now I have more to think about!
: )

Reading Elephantear's post reminded me I forgot to mention that I will make Nessie in sections. I would like to be able to move Nessie around.
Thanks again.

nmgirl, just had another thought. Idea for the tail - could you maybe hang a knee-high stocking some place fill with crete with a plastic bag over it(to keep the Crete from drying to fast) with the end of this 'perspective' tail resting in a damp sand pile where you could form the curve? this way the tail would be nice and round. After a few days of curing if the stocking 'held tight' that it was unremovable you could add a thinner layer of mix to cover the whole tail while it's now on a sand pile, one side at a time etc.? does this sound like a crazy idea or what? Wendy

wow, earthworms, that is funny. thanks for sharing it. Personally I wish I had a gopher snake that big (so he would not be filled up quick ) and put him in this neighborhood...he would never leave, lol. Can't wait to see your Nessie.

Here is a snake made out of huge tires...

Enjoy!

Lin

Here is a link that might be useful: huge tire snake

Hi...I haven't been on here in some time and thought I would drop in again. I saw this thread and thought I would share my "Nessie" with you. I made this 3 years ago for our grandson. It is made from chicken wire, moss, placed over wood to support and filled with soil. Then I planted it with thyme and sedum. It is still (keeping fingers crossed) standing and growing (well once spring comes!)as you can see in the last picture of our ice storm mid march.
Anyway here are the photo's....
First made and planted
{{gwi:133090}}

Side view, looks a little boxy till the plants fill in;
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Just taken two weeks ago after our ice storm..so far, so good...people still enjoy seeing "her"
{{gwi:133092}}

WOW! Taigen! How perfect! I like the size and the idea of it being a living planter thingy!!!