It's probably the Classic Flyers mod. I had that briefly installed, realized it actually modified my existing creatures' stats, and uninstalled it. I know this because I keep records of every stat for the things I'm breeding. After I uninstalled, there were wild flyers around the world with levels that were impossible. I just hatched some wyvern eggs that I've been holding on to for a while, and they also hatched at impossible levels, so I think I must have gotten them while the Classic Flyers mod was installed.