heffalump
(A child's name for) an elephant.
We went to the zoo and saw some heffalumps.
Something that is elusive.
figuratively, informal
With the study in this chapter, we continue the hunt for the Heffalump to answer the 'who is the entrepreneur' question [...]. The Heffalump is a character from Winnie-the-Pooh that has been hunted by many individuals using various ingenious trapping devices, but no one has succeeded in capturing it so far. All who claim to have caught sight of it report that it is enormous, but they disagree on its particularities [...].
Where it comes from
Probably a childish mispronunciation of elephant, perhaps influenced by half a lump (as in "I'll have half a lump of sugar in my tea"), coined by the English author Alan Alexander Milne (1882–1956) as the name of an imaginary animal in his book Winnie-the-Pooh (1926). The animal was not described in the book, but the illustrator Ernest Howard Shepard (1879–1976) depicted it as an elephant. Sense 2 (“something which is elusive”) refers to the fact that in Milne’s book the characters Winnie-the-Pooh and Piglet set a trap for, but are unable to capture, a heffalump.
Adapted from the Wiktionary entry heffalump (CC BY-SA 4.0).