OK, on this one we are going to have some fun! Let the accusations begin!
Do Mormons have horns?
No, in fact we don't. After many generations of converts and genetic selection, it is very rare for a Mormon to be born with horns. Those that are, have them removed shortly after birth.
Seriously? Ok, perhaps in the 1800's I could see people asking this, but anytime in the last 50 years is inexcusable. No Mormons do not, nor have they ever had horns. This was merely a contrivance of ministers who felt they were losing members to try and associate the LDS Church with Satan. It was and is a flat out lie, and anyone who perpetuates it should be suspect as a source of any truth.
Do we have multiple wives?
Are you kidding? It is hard enough to keep one wife in line!
Now to the answer. No. Actually polygamy was done away with in 1890 via what is called the Manifesto, and is contained in our Doctrine and Covenants as Official Declaration 1. A good discussion about both the practice and abolishment of polygamy can be found on the FairLDS.org website here.
There were a small number of people who were married into polygamist marriages after this date, but from what I have seen the vast majority of cases were additional wives being added to already existing polygamist relationships, and none after about 1910. I actually have a Great Great Aunt who was one of these last ones. While she died in 1977, there is still a lot of lore that surrounds it.
Today's reality is that no one in the LDS Church currently practices it, and if they are found to be doing so, they are excommunicated.
The "Mormon" groups that still practice polygamy are all apostate offshoots of the LDS Church, and have little to nothing to do with it.
I could get into the whole discussion of the whys and explanations of the practice, but that is for another article. The real point is that even among those who did within the LDS faith, I doubt any are still alive that participated as it was changed so long ago.
I heard the reason Mormon churches have steeples is that Satan will fall out of the sky and fall on it. Is that true?
No. It isn't true. Not even a little bit.
Beyond that I'm really not sure where to go with this one. I am not one to belittle these comments generally, as I wish to dispel them kindly, but this is pretty out there. I had this one asked to me many years ago, and I was just astonished at it. So, rather that mocking, I will link to an article I found that discusses LDS architecture, and in particular steeples, in some detail.
This isn't a periodical I would normally link to, but this particular article seems sound, and worth the read.
Do Mormons dance?
Other than the fact I embarrass my wife so much she has forbidden me to dance, yes Mormons dance. Quite frankly they do it a lot.
Girls almost all take some kind of dance class, and there are all kinds of dances set up as social activities for Mormon youth. Growing up there was usually at least one dance at a LDS church in the area at on a monthly basis.
If you don't believe me, look at all the reality shows on TV these days. On those dancing reality shows it is almost unusual to NOT have a Mormon in the group.
I believe this actually started more recently as Footloose was filmed in Utah, and implied the stodginess of Mormons in so doing. It was bunk.
So yes. We dance. No always well, but we dance.
So many myths, so little time. I'll stop here for the moment. I'm betting there will be some followups though.