Quantitative evidence on the associations of temperature, relative humidity, and skin temperature (ST) with respiration rate (RR) in Korean native black goats remains limited, including potential sex- and growth-stage differences. Twenty-four goats (six per sex-by-growth-stage group) were repeatedly measured on 35 dates at a single farm over 15–33 °C, yielding 840 repeated animal-date observations. Natural-log-transformed RR was analyzed using a linear mixed-effects model accounting for animal-, date-, and pen-level dependence. Within the observed conditions, each 1 °C increase in temperature and 1 percentage-point increase in relative humidity were associated with 9.23% and 0.79% increases in RR, respectively. At 24 °C, mean relative humidity, and mean baseline body weight, adult goats had higher predicted RR than growing goats, while temperature-associated RR slopes did not clearly differ by sex or growth stage. Adding ST modestly improved in-sample model fit. Model-derived temperatures corresponding to RR criteria of 40, 60, and 80 breaths/min were 21.2–23.1, 25.8–27.7, and 29.0–31.0 °C, respectively, conditional on mean relative humidity and mean baseline body weight. These preliminary population-level reference points are not validated thresholds of heat-stress severity, welfare impairment, or production loss and require external validation in larger, multi-farm studies using independent physiological, behavioural, welfare, and production outcomes.