1. Dick = Richard

2. Bill = William

3. Nancy = Ann

4. Ted and Ned = Edward

5. Nellie = Helen

6. Daisy and Peggy = Margaret

7. Sally = Sarah

8. Polly = Mary

9. Hank = Henry

10. Jack = John

11. Chuck = Charles

12. Buffy = Elizabeth

How is Polly is short for Mary?!
Going back to the Middle Ages, there weren't that many first names yet. And so many people were named Richard that everyone needed nicknames to tell one another apart. Richard was shortened to Rick, and then people would rhyme it with something else to become an entirely new name — so Rick became Dick. (And then the modern trend of being dirty little pervs meant that everyone today giggles when they hear of a guy named Dick.)
William was also a popular name in the Middle Ages, so many nicknames were born. It was shortened to Will, which turned into Bill. Rhyming nicknames strike again!
Why is the short form of Ann (or Anne) actually longer than the original name? People used to use the affectionate phrase "mine Ann," which eventually turned into "my Nan." Nickname trends of the time also had people adding "-cy" to the ends of name, which is how Ann evolved into Nancy.
Fun fact: This means that sisters Ann and Nancy Wilson, members of the band Heart, were kind of named the same thing.
Yep, you've got it: "Mine Ed" turned into "my Ned."
As for Ted, just as Richard and William were popular names, Edward was a very common name that required nicknames to be created. With names that start with vowels, people often added an easy-to-pronounce consonant, so Ed became Ted.
Yep, Nellie is similar to Ned and Nancy. Since, depending on your accent, the H in Helen might be silent or difficult to say, it was dropped and "mine Helen" turned into "my Nell" and then into "my Nellie."
It seems like there should be a long explanation for Daisy, but it's as simple as this: The French word for "daisy" is "marguerite."
As for Peggy? Well, Margaret was shorted to nicknames like Meg or Meggy, and the rhyming nickname trend turned Meggy into Peggy.
In the time of the Normans in the 11th and 12th centuries, people would often substitute the letter R for other letters (in this case it's replaced by two L's), and would add a Y to the end as well. And so Sarah became Sally.
First off, we have another case of the letter R being replaced by two L's. Then, the natural evolution of language turned Mary into Molly. And yes, more rhyming occurred, turning Molly into Polly.
No one is entirely sure how this came to be, but a popular theory is that the name Hendrick is the Dutch version of Henry" Then, Henk became a nickname for Hendrick, so English people borrowed it and eventually it became Hank.
They have the same number of letters! How does that save any time? There are multiple theories about this nickname, but here's the most likely one: Back in the 11th and 12th centuries, the Normans would have pronounced "John" as "Jen." They also added "-kin" to the ends of names as nicknames. So, Jen turned into Jenkin, which eventually turned into Jakin, and finally into Jack.
This one's a little simpler: In Middle English, Charles was actually Chukken. (Yes, really.)
There aren't even any F's in "Elizabeth"! Come on! But in this case, it's a nickname based on how a child might pronounce the final syllable — Elizabeth becomes Beth, which becomes Buff, and then becomes Buffy. (So, yes, the full title of the show should have been Elizabeth the Vampire Slayer.)