Lion was one of the motifs used in Ashokan pillars. Lion is a solar symbol and it also represents the Buddha, as he is referred to as Sakyasimha which also means a lion among the Sakyas.
Ashoka was a patron of Buddhism therefore he used many symbols that are relevant in Buddhism as motifs in the Ashokan pillar. According to Buddhist tradition, Buddha entered his mother’s womb in the form of a white elephant.