Where is the very last egg in Spyro: Year of the Dragon?

In the original releases of the game there is a bug with the egg reward from any of the 4 speedways. Sometimes you are not rewarded with an egg after winning the race/time attack.

This may explain why you are missing an egg. Unfortunately it is not easy to remedy.

The Greatest Hits, Collector's Edition and Platinum versions will not have this bug and additionally they are the solution to fixing your save file. Load your save game on one of these versions of the game and you will be instantly rewarded with your missing egg.

If you have just beaten the Sorceress boss for the first time, then you probably miss The Super Bonus Round, the final realm in Spyro: Year of the Dragon.
You can enter it after you've collected all 15,000 gems and 149 Dragon Eggs from the other realms. The portal to this realm can be found in Midnight Mountain, a short glide away from the Harbor Speedway.

In this level you can find 5000 extra gems and the very last egg you miss.

After that you should have 150 eggs and a 117% completion percentage.