The Stalker: Individual who appears the moment you start using a piece of equipment and hangs around awkwardly until you finish. May well have a deep, morose stare not unlike Pvt. Pyle's from Full metal Jacket. Will decline offers of doing alternate sets via mute silence, although perhaps because this sounds like a sex act.
The Professor: older gentleman who wears grossly inappropriate gym clothing, e.g. tweed jacket, polished black shoes, flannel trousers.
Anorexia Nervosa: AN is at the gym every time you enter, and still there when you leave. AN's bones are clearly visible at every inch of exposed flesh. Invokes sadness and, inevitably, self-loathing due to hatred of gym culture.
Grunter: individual who makes sex noises during exercise.
Cock of the Walk: spends too long with genetalia exposed in changing rooms. My rule is expose genitals for no longer than is necessary, but don't be weird about it (aka 'Towel-Changer'). Cock of the Walk (male) may have a powerful appendage, or a cheeky little acorn. No correlation. Cock of The Walk just likes it out there.