Laudator Temporis Acti

Aesop, Fable 185 Chambry (tr. Olivia and Robert Temple):

A dog was crossing a river holding a piece of meat in its mouth. Catching sight of his reflection in the water, he believed that it was another dog who was holding a bigger piece of meat. So, dropping his own piece, he leaped into the water to take the piece from the other dog. But the result was that he ended up with neither piece -- one didn't even exist and the other was swept away by the current. This fable applies to the covetous.

Κύων κρέας ἔχουσα ποταμὸν διέβαινε· θεασαμένη δὲ τὴν ἑαυτῆς σκιὰν κατὰ τοῦ ὕδατος, ὑπέλαβεν ἑτέραν κύνα εἶναι μεῖζον κρέας ἔχουσαν. Διόπερ ἀφεῖσα τὸ ἴδιον ὥρμησεν ὡς τὸ ἐκείνης ἀφαιρησομένη. Συνέβη δὲ αὐτῇ ἀμφοτέρων στερηθῆναι, τοῦ μὲν μὴ ἐφικομένῃ, διότι οὐδὲ ἦν, τοῦ δὲ, ὅτι ὑπὸ τοῦ ποταμοῦ παρεσύρη. Πρὸς ἄνδρα πλεονέκτην ὁ λόγος εὔκαιρος.

For a list of adaptations and translations of this fable, see here. Jonathan Swift's translation of this fable into Latin could be added to the list:

Ore cibum portans catulus dum spectat in undis,
Apparet liquido praedae melioris imago:
Dum speciosa diu damna admiratur, et altè
Ad latices inhiat, cadit imo vortice praeceps
Ore cibus, nec non simulacrum corripit unà.
Occupat ille avidus deceptis faucibus umbram;
Illudit species, ac dentibus aëra mordet.