The Fifth Fleet is the command of Commander Cain on battlestar Pegasus.
The Fifth Fleet is destroyed by a Cylon ambush at the Battle of Molecay some time prior to Battle of Cimtar.
Pegasus is presumed to be lost as well, but later meets up with the fleet near Gamoray (TOS: "The Living Legend").
Spoilers
- Is it by accident that the fleet has the number 5? "5th fleet" sounds as if the original idea was that the Pegasus was a second survivor of the battle of Cimtar (the only battle we know in which 5 battlestars participated); It smells a bit like a late script change that took place after someone mentioned that the battlestars at Cimtar had all already been named.
- It is at least a bit strange that the Colonials did not name their fleets by Colonies (e.g. "Caprican Fleet" or "3rd Caprican Fleet").