renaerys:

Yes! I have a theory (just fanon, of course) that her forehead symbol is something she developed over time from when she was young. As for canon, however, lot of Mito stans seem think the symbol is inextricably linked to medical ninjutsu, which I can understand because of the connection with Tsunade and Sakura. But actually, the forehead symbol is just a means to store a massive amount of chakra. 

Tsunade explicitly says she uses that chakra to power her Genesis Rebirth technique, but the underlying fact is the same: the chakra stored is in no way linked to medical ninjutsu; rather, it’s simply a massive storage of chakra to be used for any purpose whatever. Tsunade chose to use it for medical ninjutsu, as did Sakura, because that is their main power (“I used that chakra to stimulate production of various enzymes…” not “The seal and the chakra it generates are medical ninjutsu” or something along the same lines). So it makes sense that they would use it for that. But it’s inferentially possible to use that stored chakra for other means, such as sealing an entire Bijuu within yourself, because it is just that: generic chakra. I’m sure it could be used for a number of different things, actually. It just so happens that Tsunade and Sakura use it for medical ninjutsu. 

Again, this is fanon, but I bet Mito used her seal to confine Kurama inside herself. I don’t think Mito was a medical nin (and I don’t think she needed to be because we already know Hashirama was a medical nin, so he covers her and Madara for that). Rather, I think Mito probably used that extra chakra to seal things she could not otherwise seal with her limited chakra. Tsunade just appropriated the seal for other means, and I’m sure it could be used for other things, as well. 

But I bet Mito developed the seal herself. We haven’t seen anyone else with it before her, so it’s entirely possible. I bet it was a shining accomplishment of hers.