Lets see into my vast knowledge of mythical creatures for this quest of a mascot...
We could have a roc, a giant bird. It's eggs are as big as a full grown man. Maybe too big, eh?
What about a kelpie, the water horse? Hard to tame, I'd say. It lures passerbys into its lake, where they are never seen again. Whether or not the victims are drowned, eaten or kept as company, no one knows.
Gryphons are pretty cool, and it could protect our loot, as that's what wild gryphons do!
Then we've got the pixies, the sprites, the trolls, chimeras, harpies, and Wyverns are basically dragons...
Ohh! A jackalope! A rabbit with deer horns! :D and it could be an exra big one, like the size of a dog or something. Make it a mount!