Now listen while I show you, how the rest
Who call you madman, are themselves possessed.
Just as in woods, when travellers step aside
From the true path for want of some good guide,
This to the right, that to the left hand strays,
And all are wrong, but wrong in different ways,
So, though you’re mad, yet he who banters you
Is not more wise, but wears his pigtail too.

[Nunc accipe, quare
desipiant omnes aeque ac tu, qui tibi nomen
insano posuere. Velut silvis, ubi passim
palantis error certo de tramite pellit,
ille sinistrorsum, hic dextrorsum abit, unus utrique
error, sed variis inludit partibus: hoc te
crede modo insanum, nihilo ut sapientior ille
qui te deridet caudam trahat.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Satires [Saturae, Sermones], Book 2, # 3 “Si raro scribes,” l. 46ff (2.3.46-53) (30 BC) [tr. Conington (1874)]
    (Source)

Horace may be quoting or paraphrasing Stertinus here.

The odd "tail" metaphor was a proverbial expression for unknowingly being a fool. Apparently Roman children would tie sheep or other animal tails to the backs of innocent passers-by, then laugh at them.

(Source (Latin)). Alternate translations:

Now leane thyne eares, and listen well, perceave howe all be mad,
Yea those who earste to make the woorse, such mockeries have had.
Admit there be through darkesum wood a speedie footepathe way,
On ryghte syde sum, on lefte syde sum, and all do go a stray
Through wilsumnes of wildernes: the error is all one,
Though through miswandringe diverslye, they diverslye have gone.
Thou maist be mad, frend Damasipp, thou maiste be muche unwyse,
Thy mockers staringe mad also, though in an other guyse.
[tr. Drant (1567)]

Nay give me leave, and I'le demonstrate how
He who calls thee fool's as much fool as thou.
Like Trav'lers passing through a Wood, when they
Range up and down missing their ready way,
This to the right that to the left hand strayes,
One error fools them both, though several wayes.
And tho thou think'st thou'rt mad, yet even he
Is not a jot less mad that laughs at thee,
Both to Fool-coats have like propriety.
[tr. "A. B."; ed. Brome (1666)]

Some call Thee mad, but those that call Thee so,
Observe, I'll prove them quite as mad as You:
As Men that lose their ways in Woods, divide;
Some go on this, and some on t'other side,
The Error is the same, all miss the Road,
Altho in different Quarters of the Wood.
Thus as they call thee, think that thou art mad;
But those that call thee so are quite as bad.
[tr. Creech (1684)]

And they who call you fool, with equal claim
May plead an ample title to the name.
When in a wood we leave the certain way
One error fools us, though we various stray:
Some to the left, some turn to t'other side:
So he, who dares thy madness to deride,
Though you may frankly own yourself a fool,
Behind him trails his mark of ridicule.
[tr. Francis (1747)]

Hear now why those who proudly call you mad,
In reason's view are every whit as bad.
As, when bewilder'd in a wood by night
This trav'ller takes the left and that the right,
Each strays, though in a different path he strays,
Mock'd by the self-same error various ways, --
So is it here; and he that laughs at you
May wear the cap; for he is crack-brain'd too.
[tr. Howes (1845)]

Now learn, why all those, who have fixed the name of madman upon you, are as senseless as yourself. As in the woods, where a mistake makes people wander about from the proper path; one goes out of the way to the right, another to the left; there is the same blunder on both sides, only the illusion is in different directions: in this manner imagine yourself mad; so that he, who derides you, hangs his tail not one jot wiser than yourself.
[tr. Smart/Buckley (1853)]

Now listen to the reason why all those who've called you "madman" are as mad as you. As in the woods, when some mistake drives from the beaten track men vaguely wandering, one goes off to the right, another to the left, -- they make the same mistake, but in quite opposite directions; -- so think that you're mad, and that the man who mocks you is no saner than yourself, and a fit laughingstock for boys.
[tr. Millington (1870)]

Now let me show you why all who have dubbed you "madman" are quite as crazy as you. When men miss the path in a forest and go astray in every direction, all miss it equally, though one is led wrong on the right side of the road and one on the left. So for yourself, believe that if you are mad it is only in such a sense that the man who is laughing at you drags his tail also.
[tr. Wickham (1903)]

Now learn why all, who have given you the name of madman, are quite as crazy as yourself. Just as in a forest, where some error drives men to wander to and fro from the proper path, and this one goes off to the left and that one to the right: both are under the same error, but are led astray in different ways: so believe yourself to be insane only so far that he who laughs at you drags a tail behind him, no whit the wiser man.
[tr. Fairclough (Loeb) (1926)]

Now learn
Why it is that all who have fastened the name on you
Are quite as crazy as you. Just as men in a forest,
Whom confusion forces to wander away from the right path,
Will veer off, one to the left, the other to the right,
Misled by the same mistake but misled in different
Directions, so you may consider yourself deluded
To the exact degree of the man who makes fun of you,
Who is dragging a tail behind himself all unawares.
[tr. Palmer Bovie (1959)]

Now, hear why you're no madder
than all those others, who bestowed on you the name
‘insane.’ Think of travelers in a forest who get lost
and leave the proper path: one might wander over
to the left, the other to the right. They're deceived
in different ways, but it's the same mistake. Similarly,
you think you're insane, but who is any wiser
among those tail-draggers who make fun of you?
[tr. Fuchs (1977)]

Listen: here is why everyone
Who calls you mad is just as senseless himself.
It's like a forest, where people wander this way and that,
Hunting the path and never finding it, not right, or left,
Or center, all confused, all equally lost, but all
Lost in different directions. Believe yourself mad,
If you like, but as sane as the man who laughs at you
And never sees the tail tied behind him.
[tr. Raffel (1983)]

Now hear me: why all those who have
called you crazy are brainless like yourself.
As in the woods folk wander off
the true path in error and scatter
here and there, this one to the left,
this one to the right, both of them in
different directions; in the same way
you may consider yourself insane.
Yet you know full well that he who
derides you is no wiser than you
but drags a tail behind him.
[tr. Alexander (1999)]

Now, here’s how
those who call you mad are mad. In a wood,
error diverts men from the strict path, some
left, some right. They’re all wrong, each in his
way. Who says he’s right is of course wrong —
is he the one you’ll let pronounce you wrong?
[tr. Matthews (2002)]

Now this is the reason
why those who call you mad are every bit as crazy
as you are: You know how people lose their way in the woods --
one goes wandering off to the left, another to the right;
both are equally wrong, though each has strayed in a different
direction. So you may rest assured that if you're to be counted
mad the fellow who laughs at you is no saner himself.
He too has straw in his hair.
[tr. Rudd (2005 ed.)]

Now learn why all those who call
You insane, are every bit as foolish themselves.
It’s like a wood, where error leads men to wander
Here and there, from the true path, one off to the left,
Another off to the right, the same error both times,
But leading them in different directions: so know
You’re only as mad as the man no wiser than you
Who laughs at you, but still has a tail pinned behind.
[tr. Kline (2015)]