Jellyfish was written by Fredrik Dahl (firstname.lastname@example.org, fredrikd on FIBS) and Effect Software, Norway.
According to David Montgomery (email@example.com, monty on FIBS), the jellyfish username was an alias for the current release of Jellyfish itself. During 1997, jellyfish played at version 3, level 7, with a time factor of 5-8. At that level, it averaged a rating nearly 150 points higher than jf_level_five (which, as its name suggests, is Jellyfish playing on FIBS on level 5.
David tracked jellyfish's and jf_level_five's ratings, and published the results in Carol Joy Cole's Flint Area Backgammon newsletter, alongside his review of Jellyfish version 3. During the tracking period, jellyfish's average rating was around 2050 and jf_level_five's was around 1900. Although jf_level_five attained a 2000 rating before it stopped playing on FIBS, its true rating is probably a little over 1900.
Patti Beadle's site contains an archive of matches between mloner and jellyfish.