If I sound bothered by this in any way, it's because I am. In case the title isn't enough, I just want to clarify that I'm not ragging on all Sonic fans, just the ones that genuinely take everything a fictional character, Eggman in this case says as gospel. Just because he's a genius doesn't mean he knows what he's talking about, he wouldn't have been defeated by Sonic countless times otherwise. I'm willing to bet he was trying to insult him, but since it apparently needs to be said due to people taking it seriously, I'm just going to say it. Hedgehogs are not rodents, they are not related to porcupines, their teeth alone should have been proof enough. Rodents have those constantly growing frontal teeth for biting into strong material such as wood or metal much like porcupines do, while hedgehogs have sharp, almost dog-like teeth for cutting and slicing. To be a rodent, they need strong, constantly growing frontal teeth which hedgehogs, possums, rabbits, and hares all lack.

And in case anyone asks; yes, I am fully aware that echidnas don't have teeth at all, but Knuckles has teeth because he's anthropomorphic, and the reason he's often depicted with fangs is because the artists that draw or model him often use human and hedgehog teeth as references.