Ji Long Feng made no claims on having found the 'XingYi fists' (by which I assume you mean the 5 Element fists) in a cave - he claimed to have rediscovered Yu Fei's art from an old manuscript belonging to Yu Fei (as the legend goes). He makes no mention of 'XingYi fists' and nobody thinks he practiced the 5 Element fists, as they are known today.

"After Yue Fei's death, the art was lost for half a millennium. Then, during the cusp of the Ming and Qing Dynasties in Shaanxi Province's Zhongnan Mountains, Yue Fei's boxing manual was discovered by Ji Longfeng (also known as Ji Jike) of neighbouring Shanxi Province.

Like the Preface, the Ji Clan Chronicles (pinyin: Ji Shi Jiapu) describes Xingyiquan as a martial art based on the combat principles of the spear. The Chronicles, however, attributes this stylistic influence to Ji himself, who was known as the "Divine Spear" (??; pinyin: Shén Qi?ng) for his extraordinary skill with the weapon.

The master who taught Xingyiquan to Ma Xue Li is conventionally identified as Ji Longfeng himself. However, the traditions of the Ma family itself say only that Xueli learned from a wandering master whose name is unknown.
Ji Longfeng referred to his art as Liu He, The Six Harmonies.

The Preface identifies Cao Ji Wu as a student of Ji Longfeng and the master who taught Xingyiquan to Dai Longbang. However, other sources identify Dai's teacher variously as Li Zheng or Niu Xixian.
Xingyiquan remained fairly obscure until Li Luoneng (also known as Li Nengran) learned the art from the Dai family in the 19th century. It was Li Luoneng and his successors—which include Guo Yunshen, Li Cunyi, Zhang Zhaodong, Sun Lutang, and Shang Yunxiang—who would popularise Xingyiquan across Northern China."

The most likely source of the 5 fists is the Dai family for the simple reason Lines coming from Ji Long Feng that bypass the Dai family don't have them - they only have the animals. The Dai family were ancestors (through Li Nengran) of the 'modern' Shanxi and Hebei styles of XingYi.

Source:

http://en.wikipedia.org/wiki/Xingyi

Note:

"A reliable genealogy of Xingyiquan can be traced to the 18th century to Ma Xueli of Henan Province and Dai Longbang of Shanxi Province."

The Henan styles descended from Ma Xueli have no 5 Element fists, but lots of animals, ergo animals came first.