The ability of grazing herbivores to assign food types to categories by relying on certain relevant criteria could considerably reduce cognitive demand and increase their foraging efficiency when selecting among many different plant items. Grasses and legumes differ functionally in vegetation communities as well as in nutritive value. We aimed to determine whether sheep can generalize an aversion they learnt for a grass or a legume species to another species of the same functional type and consequently whether botanical family is a potential level of categorization. Over four successive weeks, 12 lambs were conditioned against either a freshly cut grass (tall fescue-Festuca arundinacea, N = 6) or legume species (sainfoin-Onobrychis viciifolia, N = 6) using a negative post-ingestive stimulus (lithium chloride) on day 1. Preference of all lambs between another grass (cocksfoot-Dactylis glomerata) and another legume (alfalfa-Medicago sativa) was assessed on day 3 by measuring their relative consumptions. Preference for alfalfa progressively became lower for lambs that were conditioned against sainfoin than against tall fescue, indicating that lambs generalized the aversion between species along some perceptual gradient and classed the considered grasses and legumes in distinct categories. Beyond this original result, the question now is to identify which specific plant characteristics or functional traits the animals rely on in order to form categories.