Ok, ok, everyone stop.
There is no possible way that the beast is Cloud. The game's scenes show this extremely explicitly. We know precisely what happened, and when. We know where Cloud was during all of the Nibelheim events, we know where and when Cloud was experimented on.
Cloud was at Nibelheim during the events five years prior to the game. He was the Shinra soldier standing guard outside the reactor when Sephiroth and Zack went inside. The flashback's events are mostly correct in terms of what happened, because Cloud was there for most of the events.
The end of the flashback is where the scenario changes. Cloud rushed in after everything went down, grabbed Zack's sword, rushed up, and stabbed Sephiroth while Sephiroth was so obsessed with Jenova and not paying attention. Afterwards, Sephiroth limps away with Jenova, and Cloud confronted him again on the bridge of the reactor. Sephiroth stabbed Cloud, then Cloud overpowered Sephiroth, and used the Masamune as leverage to hurl Sephiroth off the bridge.
At this time everybody is essentially completely worn out and half dead with blood loss or exhaustion. Zagan rescues Tifa, his pupil, but the rest of the survivors, including Zack and Cloud, are picked up by the Shinra cleanup team (led by Hojo) and experimented on. This is when Cloud (along with the other Nibelheim survivors) is rebuilt into a Sephiroth clone. Part of the reason Hojo did it was because Hojo was completely shocked that Cloud had managed to overpower Sephiroth, something he had previously thought impossible. This might be part of the reason why the two of them are segregated from the rest of the survivors and experimented on in the basement together, but there is no concrete evidence of this.
During this time, Zack recovers better than Cloud, and manages to bust the both of them out. He grabs Cloud, gives Cloud an old uniform, and the two of them head to Midgar. During the trip, Zack talks to Cloud. We're explicitly shown him talking to Cloud about what he's going to do when he gets to Midgar, but given the length of the trip from Nibelheim to Midgar, I think it's a fair bet that Zack talked to Cloud about all sorts of things, from his early days with ShinRa, to his adventures in SOLDIER.
This is most likely the time during which Cloud's "Zack Persona" developed. Cloud, having suffered extreme mental trauma through both the events that occurred surrounding Sephiroth, and the experimentation done to him, retreats in his own mind. He develops a split personality. The core of his mind hides away, and is the ghostly "child Cloud" that we control during several of the scenes when Sephiroth dominates Cloud's minds and actions. A replacement personality develops, which fuses his own memories with the tales Zack told him. This new persona serves to hide not only from the events of Nibelheim, but also from Cloud's own failures. He retains his memories of his past, but everything from when he joined ShinRa on are based on Zack's memories. Joining SOLDIER, becoming First Class, being old war buddies with Sephiroth... All of these are Zack's memories. These are supported and influenced somewhat by his own (such as allowing him to recall ninety percent of what happened during the flashback scene), but also serve to distance him from the events to a degree.
Reconstructed Memories:
The Bonus Scene from the Shinra Mansion During Disc 4:
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