Perry's Index to the Aesopica
Perry reserves 458 for the natural history anecdote about the snake called
dipsas who cheated the donkey of the gift of immortality. The story is
found in Aelian's History of the Animals 6.51.
You can find a compilation of Perry's index to the Aesopica in the gigantic appendix to his
edition of Babrius and Phaedrus for the Loeb Classical Library
(Harvard University Press: Cambridge, 1965). This book is an absolute must for anyone interested
in the Aesopic fable tradition. Invaluable.