The aim of this study was to compare within-horse changes and between-horse differences in selected metabolic parameters for twenty cold-blooded mares, of which ten belonged to a group with a history of laminitis (LG) and ten formed the control group (CG). Measurements were conducted during four seasonal sampling periods in March (S0), May (S1), July (S2), and September (S3). Changes within individual horses over the season were determined using the coefficient of variation (CV) for each animal and differences between groups were evaluated using the t-test or Mann–Whitney test, depending on normality. LG showed a significantly higher insulin CV (LG 20.06% vs. CG 7.52%; p = 0.023). For fructosamines, the CV was higher in CG (26.61% vs. 20.09%; p = 0.0312), whereas glucose (p = 0.279) and body weight (p = 0.0576) showed no group differences. Variability between horse groups within samples was analyzed using the F-test. In S0, no differences were observed for glucose (p = 0.607) or insulin (p = 0.980), and fructosamines showed borderline significance (p = 0.05). Body weight variability was higher in LG (p = 0.010). In S1, greater dispersion in LG was found for fructosamines (p = 0.016) and insulin (p = 0.0329), while glucose (p = 0.952) and body weight (p = 0.0981) did not differ. In S2, LG showed markedly higher CV for glucose (p < 0.0001) and insulin (p = 0.0007), whereas fructosamines (p = 0.282) and body weight (p = 0.7418) remained comparable. In S3, no differences were detected for glucose (p = 0.512), fructosamines (p = 0.147) or insulin (p = 0.872), while body weight variability was again higher in LG (p = 0.0046). Overall, laminitic horses displayed greater seasonal fluctuations in metabolic parameters and, in several periods, more pronounced between-horse variability, suggesting a less stable metabolic response to seasonal conditions.