Like the ship above, this was a reuse of the model labeled as the USS Melbourne.
The footage of this vessel in "Unification I" was a reuse of the footage from the Wolf 359 debris field from "The Best of Both Worlds, Part II", though the damaged sections of the hull were clearly on fire in the "Best of Both Worlds, Part II" version. The model was later reused in "Emissary" along with the Excelsior-class USS Melbourne.
Initially the ship was disabled as it passed through a cloud of tetryon plasma, but later caught up with the Yellowstone and subsequently damaged the runabout's warp drive, causing the runabout's destruction. (VOY: "Non Sequitur")
Though from an alternate reality, this ship most likely existed in the normal timeline.
In late 2372, in an alternate timeline, this unnamed Nebula-class starship docked at Deep Space 9's upper pylon, when Starfleet decided to leave DS9 and the station was taken over by the Klingons. (DS9: "The Visitor")
Although not identified visually, it is possible that this ship is the otherwise-mentioned USS Lexington, which was referenced in comm chatter during the engagement. Some prints of Star Trek: First Contact do not include shots of this vessel.