After discussing the matter in another thread, I'm certain that all Mangekyou Sharingan techniques had their names long before any modern Uchiha unlocked them; and I believe the Sharingan itself can be likened to the Zanpakuto of Bleach in a few ways.
First of all, most every Sharigan jutsu is related to ancient legends and stories found in our world. Izanami, Izanagi, Amaterasu, Tsukuyomi, and Susanoo, they are all names from eastern legends. Considering the weight and level of importance given to these names, they surely weren't decided by any random character, they were given the name specifically. Kakashi and Obito didn't coincidently name their MS technique by the same name, it was given the name Kamui long ago and they simply knew it somehow.
Who named the Jutsu?
How Sharingan could be likened to Zanpakuto:
Summary:
First of all, most every Sharigan jutsu is related to ancient legends and stories found in our world. Izanami, Izanagi, Amaterasu, Tsukuyomi, and Susanoo, they are all names from eastern legends. Considering the weight and level of importance given to these names, they surely weren't decided by any random character, they were given the name specifically. Kakashi and Obito didn't coincidently name their MS technique by the same name, it was given the name Kamui long ago and they simply knew it somehow.
Who named the Jutsu?
Now who would be the best authority to name such powerful and meaningful jutsu? Would a demi-god suffice? Surely Indra, the first Sharingan user in the Narutoverse, would be worthy of naming these jutsu; and I believe he did. Kaguya herself knew the name of Amaterasu, and surely she lived to see Indra develop, so its quite likely the case. Being its first user, Indra was most likely was able to unlock some, if not all, jutsu available to a modern Uchiha. If he had access to all of them, or at least knew about all of them, surely he would be the best authority to name them.
Assuming Indra unlocked and named every jutsu the Sharingan could possibly unlock, surely he would pass that information down? Surely the names of all these jutsu are written on the Uchiha tablet? But if he didn't do that, and if they weren't passed down via a secret scroll of some sort, how would modern Uchiha possibly know their names? This is where the Zanpakuto comparison comes in.
Assuming Indra unlocked and named every jutsu the Sharingan could possibly unlock, surely he would pass that information down? Surely the names of all these jutsu are written on the Uchiha tablet? But if he didn't do that, and if they weren't passed down via a secret scroll of some sort, how would modern Uchiha possibly know their names? This is where the Zanpakuto comparison comes in.
How Sharingan could be likened to Zanpakuto:
Perhaps the Sharingan eye and the related jutsu are similar to the relationship between a Shinigami and his or her Zanpakuto. We know that the Shinigami needs to be in tune with his or her sword to be able to hear its whisper and learn its true name; without that information, they can never unlock its true power. Perhaps the Sharingan and MS are comparable to this, once you unlock the latter stage of the doujutsu, you become capable of harnessing its full power by learning the names of its strongest jutsu.
Now I'm not saying the Sharingan whispers to the Uchiha or anything, (I'm certainly not that crazy -_-), but perhaps along with unlocking that eye, they are somehow granted the knowledge of the jutsu that they just unlocked. I mean take Sasuke for instance. How the heck did he know he could actually utilize Amaterasu during his fight with Bee? He obviously knew the name from Itachi, but once he unlocked MS, how did he know that ability was now his to yield and how did he know he was even capable? Sharingan Zanpakuto of course! This would explain the fluidity of MS users instantly knowing the names of their jutsu without any real explanation.
Now I'm not saying the Sharingan whispers to the Uchiha or anything, (I'm certainly not that crazy -_-), but perhaps along with unlocking that eye, they are somehow granted the knowledge of the jutsu that they just unlocked. I mean take Sasuke for instance. How the heck did he know he could actually utilize Amaterasu during his fight with Bee? He obviously knew the name from Itachi, but once he unlocked MS, how did he know that ability was now his to yield and how did he know he was even capable? Sharingan Zanpakuto of course! This would explain the fluidity of MS users instantly knowing the names of their jutsu without any real explanation.
Summary:
Perhaps unlocking the Mangekyou Sharingan unlocks some sort of connection to Indra that teaches you the name of the Jutsu you just unlocked. Indra most likely had access to, or was aware of, every Sharingan jutsu that ever came to be. He most likely named each of them himself, and is why they were all given such legendary names. Sharigan users like Itachi, Obito and Kakashi did not simply name those jutsu themselves, they were aware of the names by some other means.
