I was reading 2 Nephi 21:8 with my daughter, and she struggled with a few words. She read,
8 And the sucking child shall play on the hole of the asp,
She said, “It says, A-S-P”
I said, “Asp. Do you know what an ASP is?”
“No.”
I said, “It is a venomous snake.”
“Oh”. She continued with verse 8.
and the weaned child shall put his hand on the cockatrice’s den.
“What’s a cockatrice?”
“I have no idea. Let me look it up.”
So, I pulled out Google on my phone, and it came up with a Wikipedia definition.
A cockatrice is a mythical beast, essentially a two-legged dragon with a rooster‘s head.
While I know that this verse is also found in Isaiah, why on earth did Joseph translate this as a mythical animal?
Wheat & Tares 
Probably because Joseph and others in the 18th century didn’t have a problem with the many mentions of “dragons”, just like those who wrote it in the first place; they didn’t have sufficient words to make better differentiations between “serpent”, “small poisonous snake” (cockatrice), or “large scary lizard/snake” (dragon).
A cockatrice is a cock’s egg hatched by a snake or a lizard.
It turns those it touches to stone.
It is the size of a rooster.
But in the original is is there for poetic parallelism.
It is a word for a deadly snake not found in the Anericss.
http://www.biblestudytools.com/dictionary/cockatrice/
Much of it is poetic form over substance.
Which makes sense.
Joseph Smith did not translate the plates so you could/should have asked the question differently.
Now if only there were a device that might be put into a hat to provide us with a translation of this concept…
Hoffbegone, Your comment is vague. If Joseph didn’t translate the plates, what did he do? What is the correct question?
I saw another bible commentary said a cockatrice was a type of serpent and this is a mistranslation of kjv. One would think the the most correct book on earth would have fixed this mistranslation. Kjv translators didn’t know what snake it was and used cockatrice in stead of the correct snake. Cockatrice is not in the original Hebrew.
Pethen – one type of poisonous snake, translated as “asp” by the KJV translators, and Tsepha – a brood (hence later in Isaiah, an egg) of that or another type poisonous snake.
Purely poetic parallelism in the Hebrew, as Stephen Marsh points out. But the translation of Tsepha as “Cockatrice”, the mythical creature noted in this thread, is unjustified by the Hebrew text, and wrong.
A concrete example of how the translation from brass plates into “reformed Egyptian” into the letters on a scroll on a brown Seerstone by the gift and power of God somehow replicated the exact same translation error of the KJV translators, using a word “Cockatrice” that did not exist in New England vocabulary except as part of this same scripture.
Yup.
A jackalope, then. Cool!
Ok, I learned what a cockatrice is. Now what part is the den?
I guess it’s the cockatrice’s home. Never mind. Weird scripture. Where do mythical beings live? Mythical dens I suppose.
Probably more like an esquilax.
Imagine my disappointment to look up esquilax and find……
I once had an Institute teacher whose opinion about the Isaiah chapters in the BoM is that Joseph and his scribe basically copied them out of the KJV except in places where he had a strong impression (via the stone[s] or however, I guess) that some small wording needed to be changed. I suppose that makes as much sense as anything. It would save Joseph a lot of time, and would also explain why Mormon could repeat nearly verbatim parts of some of the epistles of Paul.
Looking into the context of Jeremiah 8:17. Its not necessarily a mythical beast,but rather a result of an action.
A snake makes a hens nest its den,and lays her eggs. The snake leaves and the hen hatches the eggs. The snakes eat the chicks. The instinct of the hen protects the small snakes allowing them to grow . Thus to simply say the hen hatches snake eggs. The snake did this for convience in order that her cluch has a better chance of survival.
Cockatrices