Fertility Beasts are entirely male (or nearly; they are magical creatures, and occasionally females do turn up) kindreds who continue the species by breeding with a large variety of similar kindreds. The two best known fertility beasts are Minotaurs and Unicorns, but there are others.
Fertility Beasts seem to be perfectly ordinary examples of their mother's kindreds until they reach adolescence, at which time they morph over a period of weeks or months into their true forms.
Fertility Beasts sire daughters as often as they do sons, but their daughters are not distinguishable from their mother's kindreds in any way, though they do tend to have somewhat higher than normal attributes.
Fertility beasts breed with kindreds more or less similar to themselves; Minotaurs breed with humanoid mammals, Unicorns breed with hoofed quadruped mammals. Fertility beasts tend to produce very strong pheromones to aid them in this process.
Fertility Beasts inherit some characteristics from their mothers; a Minotaur born of Trolls will be larger than one born of Dwarves. Fertility Beasts tend to be drawn to females who are significantly different than their mothers, though there have been cases where this has not been true; there are tales of a family of Unicorns who bred with nothing but buffaloes for several generations, and became significantly massive and shaggy in the process.