Duathlons are most similar to triathlons, with the key difference being the lack of a swimming leg. Another sport derived from triathlon is aquathlon, which combines swimming and running but has omitted the cycling part.
Duathlon
Duathlon is an athletic event that consists of a running leg, followed by a cycling leg and then another running leg in a format bearing some resemblance to triathlons. The International Triathlon Union governs the sport internationally.