"What goes on four feet in the morning, two feet at noon, and three feet in the evening?" (H&P give essentially the same version, p. 693, but with "legs" in place of "feet".)
Apollodorus' version is the more widely available. It runs as follows:
"What is that which has one voice and yet becomes four-footed and two-footed and three-footed?"
Also important is Athenaeus' somewhat fuller version, available at a Sophocles website, as follows:
"A thing there is whose voice is one;
Whose feet are four and two and three.
So mutable a thing is none
That moves in earth or sky or sea.
When on most feet this thing doth go,
Its strength is weakest and its pace most slow."
Besides this relatively comprehensive version, there is also evidence for a shorter version, consisting of just one dactylic hexameter line, as follows:
"A thing there is whose voice is one;
Whose feet are four and two."
One possible answer to this form of the riddle is "a pastoral society" (i.e., one in which humans and their animals live in close association with one another). Such a formulation of the riddle is important at various points in Oedipus Rex. For example, the plague is described near the beginning of the play (H&P, p. 707, lines 24-25) as affecting both the flocks and women of Thebes. Also, it is eventually the two shepherds (from Corinth and Thebes respectively), who have lived in close association with their flocks (H&P, p. 742, lines 1082-1090) who eventually provide the key evidence for explicating Oedipus' background.
You that live in my ancestral Thebes, behold this Oedipus,- him who knew the famous riddles and was a man most masterful; not a citizen who did not look with envy on his lot-see him now and see the breakers of misfortune swallow him! Look upon that last day always. Count no mortal happy till he has passed the final limit of his life secure from pain.