Based on his analyses of lynx and brown bear teeth, Björn Kurtén coined the concept of ‘realisation threshold’, the smallest size at which a tooth can form and erupt properly. Kurtén found the smallest sizes of the studied lynx and bear teeth to be 2.5 mm and 3.5 mm, respectively, much greater than for example the smallest rodent teeth known to readily erupt. A recent study comparing developing teeth from shrews to elephants suggested a relatively unchanged theoretical minimum tooth size for mammalian teeth. Together, these studies have left open the question of whether realisation thresholds of teeth are larger in larger mammals than in small ones. Here, we followed Kurtén’s line of thought and compared the sizes of teeth that are variably present in dentitions, and therefore likely cross the realisation threshold only occasionally. First, we show using published reports that variably present teeth are relatively small in large mammals, but larger than the previously suggested theoretical minimum tooth size. Next, we examine the canines of mares, which are known to be variably present. We report one canine that, compared with information found in the literature, is by far the smallest compared to the body size. In conclusion, whereas the variably present teeth tend to be larger in large mammals, there may be overlooked potential for large mammals to develop very small teeth. This information can be helpful in extrapolating findings from common small model organisms, such as mice, to larger mammals, including humans.