***(Badum, just a heads
A bit late to this, but I think that the Tantabus was around for a lot longer than that...in fact, I got a whole freaking theory/headcannon about it.
While Luna *did* say the Tantabus was her own creation made to punish herself for Nightmare Moon's misdeeds, she never said she had created Tanti after Twilight beat her...or any specifics on when Tanti was created, for that matter!
With that in mind, my theory/headcannon on this is that Luna created her LONG before she had actually became Nightmare Moon in the first place, most likely back when she and Celestia were both fillies.
The Tantabus was probably originally created as some kind of "imaginary friend"-type creature for Luna to have fun with when she was sleeping, and as her tastes matured, Tanti didn't go away, instead she provided those newer tastes for her in the dream realm so Luna could still have fun with her.
After returning from banishment from the moon and being turned back into Luna again, Tanti was forcefully repurposed by Luna to constantly punish her for what she did as Moon, and eventually, Tanti became corrupted by the nightmares Luna made her create, and decided to go into other ponies dreams to corrupt them.
After the Mane 6 helped Luna defeat the Tantabus, she absorbed her back into her own mind. Afterward, Luna not only had a peaceful sleep from that moment on, but Tanto was back to her old self again, ready to give Luna some great times in the dream realm again, just like before she was corrupted.
...I spent, like, half an hour on that. I had no idea I had sunked that much time into this comment until I looked at the clock.