Dragon and Tiger
Dragon and Tiger joining their essences in the alchemical tripod.
The dragon (a symbol of the Yang, male principle) is mounted by a girl (True Yin within Yang), and vice versa the tiger (a symbol of the Yin, female principle) is mounted by a boy (True Yang within Yin).
Source: Xingming guizhi (Principles of Balanced Cultivation of Nature and Existence).