The closest parallel to this entry (containing both of the etymologies offered here) is in an anonymous Etymologicum published in Anecdota Graeca (Cramer) 2.436.20.
[1] An absurd etymology based on the similarity between the words 'intestine' (e)/nteron) and 'different' (e(/teron). The same etymology is also offered in entries on e)/gkata ('innards') in Etymologicum Magnum 310.9 and Etymologicum Gudianum 394.19.
[2] Another etymology, as if e)/nteron were a combination of words for 'within' (e)nto/s) and 'flow' (r(ei=n). Also found in Etymologicum Magnum 344.32. Cf. the similar etymology in Etymologicum Gudianum 480.13, where the word is derived from 'fitting' (a)rhre/nai) within.