Consulting the source paper for the rectum-eating eel (Siu Fai Lo, Sin Hang Wong, Lok Sang Leung, In Chak Law, Andrew Wai Chun Yip, Traumatic rectal perforation by an eel, Surgery, Volume 135, Issue 1, January 2004, Pages 110-111) where the fish is not identified further than to “eel”, it appears from the photograph there, which is very small and in low resolution, that this is not an eel at all, but more likely a swamp eel, apparently Monopterus albus, a common food fish in China where it is sold alive in the markets. This identification is suggested by the very slender tip of the tail, and somewhat inflated gular region. Thanks to Ralf Britz, expert on this order of fishes, the Synbranchiformes, for inspiring me to look at the original paper and first suggesting the identification. The swamp eel portrayed here, was never inside a human, though:
Monopterus albus, preserved. Photo Sven O Kullander, CC-BY-NC