That'll stop 'em. 
A roughly egg-shaped thing that is not really an egg (or just not a dragon egg) - LOTS of possibilities there, and easy to play on the Players' meta-expectations, appearing to follow a well-known formula but then taking a hard left turn. (And that way, even if they DO read this, they will have to keep second guessing themselves, and where the plot is going.)
Maybe it's just not Animal or Vim, maybe it's Elemental? And/or high MR. But something easy to solve, just not obvious, that's my preferred approach - adds to the "D'oh!" factor when they DO figure it out. (Or some obscure and ancient magical item of power - damn, you can't walk but trip over one it seems.)
Or it was an egg, but has been changed, enhanced/enchanted/changed, or is both - or could be, depending - an egg to be hatched or something more, depending how it's handled.
But meanwhile, who has it, or knows of its location? Maybe a town has it, and they demand a trade, to solve other problems they have?
Some noble owns this land and is in charge of its welfare and concerned with its prosperity (and/or his own skin) - what is he and/or his doing about it? What are his neighbors doing now that he's busy? What is the local Church doing? The Locals themselves?
(And once the magi have it, if it is NOT organic, maybe it is part of a set, and the other one is... still with the dragon? Story seed, for later, that's sure to eat at one or more of the PC's...)
While "dragons" is the expected encounter, anything that comes with dragons, or that they leave behind them, is valid - from dung to heartbreak, lawlessness or desperation, treasure hunters or would-be heroes, predators or refugees - any of those is a mini-adventure seed.
If a dragon does what they do best, they are burning what's handy, eating flocks and virgins and hoarding/raiding treasure - any of these that slipped thru the cracks (for the time being?) is fair game.
But I will tell you this - in my experience, the best idea is not to nail it down, not yet. Have "something" - and not decide, one way or the other, until you have to, until the Magi actually have it in their hot little hands and are in a position to crack its secrets. Once they do, and long before that, you as SG will have heard all their fears and hopes and "what if's" - and have a MUCH better idea of what will make a good story. And since no one knows what it is until a PC actuall finds out, delaying changes nothing in their eyes - so long as you can spin a good tale. 
