The Artscroll gemora says," The men of the great assembly prayed and fasted for the end of evil inclination toward idolatry.The likeness of a fiery lion cub emerged from the holy of holies.A prophet said to Israel.This is the evil Inclination of Idolatry."Why a lion cub? Thankyou
yale siegel, St Louis Pk Mn U.S.A.
The Maharsha in Yoma (69b) says that this is because the Mazal of the month of Av, during which the first Beis Hamikdash was destroyed (before this incident happened), is that of a lion (Leo). Being that the destruction of the first Beis Hamikdash was primarily because of Avodah Zarah, the symbolism of the Yetzer Hara was represented as the Mazal of Av.
All the best,
Yaakov Montrose