The implication is that, upon telling the truth, one must be ready to gallop his/her horse away, any moment because, people may find such justness unacceptable and resort to violence in response. Being honest may get you in trouble, thus, in order to avoid further problems, one must distance himself/herself from the crowd as soon as possible. Is there a proverb or idiom in English identical in meaning ?
Not identical but another related phrase is "Sticking your neck out"
"If you stick your neck out, you say something which other people are afraid to say, even though this may cause trouble for you."
Similarly (and possible closer to your idiom)
if you Put your head above the parapet or stick your head above the parapet, you give an opinion in public about something that may cause people to attack or criticize you.
(Parapet is a defensive wall, sticking your head above it might get it shot off)
In both cases, they imply that you have to be brave (or expect to be hurt) if you are speaking uncomfortable truths.
John Ray, A Collection of English Proverbs (1678) lists an English proverb that expresses much the same idea:
Truth hath a good face, but bad clothes.
In more recent times, this proverb has appeared as "Truth has a good face but ragged clothes." T.F. Thyselton Dyer, "Proverb Lore," in The Sunday at Home Magazine for Sabbath Reading (September 30, 1882) offers this explanation:
Again it is often said that:
"Truth has a good face, but ragged clothes,"
the meaning obviously being that although truth of itself must ever be admired, yet persons often suffer loss of some kind or another through their conscientious dislike of saying anything that even verges on falsehood.
Numerous proverb collections include an entry for the following proverb:
He who speaks the truth must have one foot in the stirrup.
Advice often given to those who serve a U.S. administration is "keep your bags packed." The idea is that while one's life may not be in jeopardy as a consequence of speaking truth to power, one's job may well be.
This quotation can be found in Steering the Elephant, ed. Rector and Sanera, p. 61, 1987.
I don't know if it's common enough to be called an idiom, but the advice
Tell the truth and duck
comes pretty close. This is a bit more of an active exhortation than the original saying (telling the truth is encouraged), but the implication that truth-tellers should be prepared to evade violence is the same.
This advice was widely attributed to Pulitzer Prize-winning journalist Les Payne, especially after his death in March of this year (see, for example, Payne's obituary from National Public Radio and a press announcement when he won a lifetime achievement award from City University of New York in 2009), but it has seen usage outside that context with various attributions, as well. For example:
A Wall Street Journal editor once advised me not to say anything about my [Tourette's Syndrome] symptoms and to hide them if I could. I discarded his advice in favor of an old Slavic proverb: Tell the truth and duck.
—James A. Fussell, "Rarely have I had to duck", The Disabled, the Media, and the Information Age, Jack Adolph Nelson, ed., 1994
“I subscribe to the Murray Kempton philosophy: ‘Tell the truth, and duck.’”
—Timothy Nolan, quoted in "Catholic playwright in search of the truth" by Jerry Tallmer in The Villager, Aug. 13-19, 2003
Another theme I heard again and again is that we must have courage to say the hard things. As Dr. Tony Byers told us his grandma said, ‘tell the truth, then duck.’
—Anne Tomkinson, "Let's Get this Party Started", The SHRMBlog, October 26, 2017
It's ironic you're asking this question on the TRUTH bowl, because you should take our name to heart and tell the TRUTH (then duck).....
—"The Truth Bowl: I Have An STD, Should I Tell"? Christian News Before it's News, May 31, 2012