One ultimate weapon that exists in “Final Fantasy VII” can actually be seen, but not gotten, extremely early in the game. This weapon is Cait Sith's HP Shout. This megaphone weapon is extremely accurate in it's name, and is hinted at from before the party has even left Midgar for the first time.
When in the Shinra building for the first time with Barret and Tifa in the party by default, there are many floors that the party must traverse. On one such floor of the building, there are a large amount of lockers in the center of the room. When searching these lockers, some items can be found, as well as some dialogue stating that there is a megaphone type object in one of the lockers, and that it would be useless to take.
This weapon talked about extremely early on in the game can later be gotten as the HP Shout, Cait Sith's ultimate weapon. Unlike most ultimate weapons that can be gotten at almost any point after certain story events, this weapon can only be obtained at one specific time, during the assault on Midgar near the end of disk 2.
The weapon will be in the same locker as from the beginning of the game, all the player must do is pick it up. Though because heading to the Shinra building is entirely optional at this point, it can easily be missed. However, the player can always return to Midgar later on if he or she previously missed the item. Much as the name suggests, the HP Shout's damage is based off of Cait Sith's health. The more HP he has, the more damage he does, which is also the same basis behind Cloud's Ultima Weapon.