f you had an empty Dinosaur nest of the correct type in your Garden, then the Water Dragon would have dropped A Diplodocus Egg for you!
I have no idea how to do this. I am sure someone has posted this before, but aware of it now as it is coming up so often. Thank You

If you want to do the Dino challenge, then you can have a look at the wildlife guide "Dinos" with the most important information.

For example you would first a nest build by Diplodocus. If you attract then Water Dragons you can get an egg. This also changes every week and is also different for others. In my case, I would have to attract/spot raccoons to get an Diplodocus egg.

Freedom Fairies United (FFU) have a Dino guide on their website.

Thank you Lesley. Shall take a peruse

I think...that at one time...if you had not done the dinos, you could collect a free diplodocus nest. If you wanted to start...grab the nest and leave it in your garden. Then if you attract five or spot five of the critter of the week...which is apparently water dragon for you...you will get an egg to hatch. After it hatches, your nest can still be filled with an egg. There are five uses for each nest, just like with the chicks.

If you go to the dinosaur tab...and read along on the left side...there still is the option to click on a link for a diplodocus nest if you have not already claimed it. One per garden. The nest can be repaired by spotting a diplodocus in another garden. And then it will be good for five more times.

This is what the wildlife tab has to say...
Dinosaurs hatch from Dinosaur eggs, so the first thing you need is a Dinosaur nest in which to hatch them.
You can get your first ready-built Dinosaur nest for free by clicking here.