I tried to common potion on my Hydra tigrean and I got this....Invalid Combination! What does it mean?

Unfortunately it's glitched right now, so you can't use it. ^_^;

If you want your Hydrus Pet Common, Try Disowning it and quickly adopting it again, it should revert it back to Common Disclaimer! This only works with Hydrus Pets ;)

Or you could just file a ticket and ask a UA to make it common.

At least that's what gunblade posted awhile ago, I think?