Detection of irregular gait (IG) during endurance competitions is based on a brief subjective veterinary examination. Horse elimination due to IG is common and may elicit competitors’ complaints. We hypothesized that detection of IG by objectively assessing motion asymmetry can be performed during endurance competitions. The aim of this preliminary study was to start investigating the feasibility of detecting and quantifying IG with a portable inertial sensor-based system (PISBS) during endurance races. Horses participating in two qualifying endurance rides were simultaneously and independently evaluated by Fédération Equestre Internationale veterinarians and the PISBS, and the results were compared after the rides. Asymmetric vertical displacement of the head and/or pelvis measured with the PISBS indicated severity of IG and identified the affected limb(s). Veterinarians and competitors answered questionnaires about the use of the PISBS. The PISBS detected IG in 21/22 horses (48/70 evaluations). Significant disagreement between the PISBS and veterinarians was detected. Disagreement between the PISBS and veterinarians was no longer detected after reducing the sensitivity of the PISBS by reclassifying horses with mild IG as sound. Competitors and veterinarians had favorable impressions about the use of the PISBS but recommended reducing instrumentation time and trotting distance for expediency. Inherent human limitations can explain the lower sensitivity of veterinary evaluation relative to PISBS evaluation. Simple methodological changes are likely to address the issues raised in this study by competitors and Fédération Equestre Internationale veterinarians. Additional studies are needed to complete the feasibility assessment of objective detection and quantification of IG during endurance competitions with a PISBS.