It is not a mistake but a leftover from the arcade version of UMK3.

Doesn't seem like a mistake to me. Seems more like a sneaky trick to get more people to spend their quarters in the arcade machines, fruitlessly trying to fight or unlock Rain.

Yep,it's like the Kano Transformations in MK2 audits or the Johnny Cage transformations in the MK3 audits,indeed is till have an old xerox page with Rain's movelist,obviously fake.

What are
'Watchdogs'.

Rain was created to be a red herring, a character shown in the UMK3 attract mode but didn't actually exist. Ed Boon did an interview before where he said that Rain was put in the game by him just to trick the players. People searched high & low for Rain, especially when there was a rumor you could fight Rain in the Graveyard. The rumors got to Boon & Tobias so they put Rain in the game as full-fledged character in the home releases of UMK3.