How big?
I can do a 20 inch egg but I won't.:)
I use acrylic light fixtures as molds. I have them from 6 inches to 18 inches.
I do two thin halves with strong fiber mud then join these together to make a ball and then cover with my finish coat.
Elongating one half to make it egg shaped is pretty easy.
I did a small 10 inch one of these Easter egg style with a colorful slurry coat, did a cracked open look on the small end and planted it with garlic chives.
Gaudy but cute, it lasted only 10 minutes at my garage sale.

I'm planning another but with a happy face on it. The chives make very cool hair. I'm also working on one with a cat face using copper wire as whiskers.

I still have 20 half balls to join and finish. i make them when ever I have a few minutes. I cover the acrylic sphere with plastic wrap and slap on the mud.

Sybil,
Sphere or dinasour egg can be accomplished and if you have done leaves you are no longer a beginnner here. Check out the sphere discussions and see the wonderful work that has been done by others there.
This is about a 16" sphere I made it by drawing with permanent marker on the inflated ball I was using. That way I would not waste materials covering the opening. You can make the opening any shape you want that way and do not have to even consider cutting through your hardened shell later.
If you are concerned that a sphere is too tough (it isn't) but next to this one is a faux rock, you can hardly go wrong with that, there are rocks so many shapes.
Happy mudding,
Dena

Here is a link that might be useful: sphere and faux rock

Anyone remember the cute dinosaur hatchling, the little monster, that Peak made last summer? If you haven't seen it, I think it is time to revisit some of Peak's creative talents. Check out that last picture! It still makes me laugh...

Here is a link that might be useful: hatchling

Wow, I'd forgotten that guy. So lifelike. Ok, maybe not having a live dinosaur/dragon around is a good thing, but it reminds me that I miss having Peakpoet around the forum.

Sigh.

Perhaps you could adapt this rocket guy's technique to a big egg. He shapes styrofoam, then covers it with fiberglass.

Here is a link that might be useful: Vatsaas brothers rocket modeling