He has been his steed from the time before he became a Knight of Daventry. Triumph is a giant gerbil. He is the third of three gerbils to share this name. It's possible that most of them died between the fifteen or so years between the Knight Tournament and his adventure in the Enchanted Isles (KQGS).
There is a memorial or grave to remember Triumph (or the Triumphs) at the Overlook looking over Castle Daventry.
Behind the scenesEdit
"Triumph" is a common theme mentioned older King's Quest lore, especially in the manuals, which discussed the ultimate goal was to 'triumph' in one's quest. In KQ8, Connor is told that only with the mask returned will he triumph.
Triumph is not a horse, but some kind of weird giant rodent or marsupial like creature, he is referred to as a 'gerbil' in one line. He looks a bit like a giant hamster, a capyabara or a wombat, with rabbit-like ears (a King's Questions answer states that wombats have never been in KQ, this might have changed, with the introduction of this strange creature). Fifi le Yipyap threatened to feed Valanice to a three hundred pound hamster, now this might actually be possible punishment, as Triumph is a flesh eater who loves giant steaks.
Graham refers to Triumph as Triumph the Third in Chapter 3. This might have several meanings, the first is that it could be the third Triumph that Graham has raised since child hood. The second it could be a reference to the 'third chapter' of the game.