In China's traditional culture and art, the fish and water designs are symbols of prosperity and harvest. The character for fish sounds the same as that for surplus and is another measure of a good year and a good life. And so Beibei carries the blessing of prosperity. The ornamental lines of the water-wave designs are taken from well-known Chinese paintings of the past. The “carp leaping over the dragon gate” is a traditional allegory of following one’s dreams and achieving them. The patterns from Beibei’s headgear comes from artifacts unearthed at Banpo, site of a Neolithic village of the Yangshao culture.
There is an old Chinese tradition of spreading good wishes through signs and symbols. Each of the five Olympic symbols has a rhyming two-syllable name which is a traditional way of expressing affection for children. When you put all their names together -- Bei Jing Huan Ying Ni -- they say "Welcome to Beijing." Beibei is strong in water sports, is known to be gentle and pure and she represents the blue Olympic ring.
Related post: Yingying, The Yellow One. And here's a selection of official Beijing Mascots.