According to the Oxford English Dictionary, the nature of the beast first appeared in the 1600s:
colloq. the nature of the beast : the (usually undesirable) inherent or essential quality or character of a person, event, circumstance, etc.
1678 J. Ray Coll. Eng. Prov. (ed. 2) 77 It's the nature o' th' beast.
John Ray's Collection of English Proverbs was a collection of proverbs from different languages, as well as a list of words, a fact noted by the title page shown here. However, it seems very possible that the phrase predates this notation. The phrase appears in a dictionary entry for nature, and the nature of the beast was used to give an example sentence.
It is possible that the phrase was well known enough that, by the time Ray compiled his list, it was an accepted idiom. However, his work is the first written usage, so we can definitely note the point at which the idiom existed. Because this is the first written usage, however, we cannot derive its origins--the phrase was written, but its precise origins were not.
That being said, the OED marks usage of beast which are related, and possibly point to the history of the phrase. In the entry of beast, two definitions are:
In early times, explicitly including man. Obs.
In later times, applied to the lower animals, as distinct from man. (First usage noted is in 1616)
The animal nature (in man). (first usage noted 1667)
The phrase the nature of the beast, having been recorded in 1678, may have been pulling on the first noted definition. That is, the nature of the beast was the inherent nature of man; that deepest essence within him. The later definitions may also work (as a beast is lower than man, "the nature of the beast" is the lower qualities within a man). The old usage of the term beast would explain the origins of the phrase--it was a normal definition of beast, which literally described the inner nature of man.
Fanboi pre-dates both Avril Lavigne's "Sk8er Boi" song (2002) and Apple's iDevices (iMac, 1998), however usage increased dramatically since the 2007 release of the iPhone, and it is now often used a a put-down when talking about zealous Apple fans.
Usenet
The excellent Technologizer post says "fanboy" originates from the comic collector world, and the earliest references to "fanboi" I can find in Usenet are also from comics groups.
In a 9th October 1996 post to rec.arts.comics.misc, The Comics Journal wrote:
With our renewed dedication to running more reviews, we'll surely end up
dealing more with the mainstream in terms of general reviews. We're also
dedicating a new nearly-every-issue column to the mainstream called "The
Fanboi Politik" by Ray Mescallado. I'm sure many opinions and critical
views will be heard on mainstream works in coming months.
Two days later, PatDOneill replies, showing from the start the term has a somewhat negative flavour:
Oh yeah--that title surely indicates what an even-handed approach it's
going to take.
Migawd--you can't even title a column about mainstream comics (that you're
touting as being written from an appreciative standpoint) without
insulting the people who are fans of such work.
How about if WIZARD retitles its column about alternative and small-press
titles as "The Nihilist, Black-Clad Claptrap"? BTW--ours runs EVERY issue.
Ray Mescallado's signature in a 22nd November 1996 rec.arts.comics.dc.lsh post was:
-- Ray
--
**************************************
* Ray Mescallado * "The politics of failure have
* fan...@avalon.net * failed. We need to make it
* http://www.avalon.net/~fanboi/ * work again." - THE SIMPSONS
**************************************
The post was asking the Legion of Super-Heroes "fandom" their opinions of the comic for the Comics and Animation Newswire. He wrote a lot about "fanbois".
Urban Dictionary
The earliest Urban Dictionary definitions aren't until 2003, but both clearly have negative meanings:
fanboi
Someone who is hopelessly devoted to something and
will like anything associated with their particular thing.
That damn fanboi only likes that game cause Capcom made it.
The only reason he bought that car is cause he is a Japanese fanboi.
Source: Jevin, Jun 19, 2003
fanboi
Alternate of fanboy.
You must be either retarded or a fanboi.
Source: loser, Apr 7, 2003
iPhone fanbois
More recently, both fanboy and fanboi are used to describe zealous fans of technology and products, in addition to comics, games and films. The use of fanboi is especially used to describe ardent fans of Apple products, and in particular appears to have increased dramatically around the 9th January 2007 announcement and 29th June 2007 release of the iPhone.
The Register is an IT news site particularly fond of iconoclastic slang. A quick tally of their 116 articles including the word fanboi show 111 are in the context of Apple/iPhone/iPad/iMac/Mac/MacBook. (The others are Java (x2), Rackspace, AWS, and e-voting.) The earliest was published in an article titled "iPhones, iPhones and more iPhones" on 13th July 2007, and is a round up of comments following the recent US release:
I _heart_ Apple
..........Insert sycophantic drivel here..........
Apple Fanboi
Google's Insights for Search backs this trend up. The main peak begins in November 2006, soon before the iPhone's official January 2007 announcement, as hype around Apple's rumoured iPod-mobile phone was "reaching fever-pitch".
Best Answer
To spin a yarn is an idiom/metaphor for telling a long tale. From Etymology online [bold is mine]:
So it is a play on words.
If thought can be seen as a thread of conciousness, then the story-teller is spinning long threads together into yarn, which is spun thread.
(source: interweave.com)
Robert Bringhurst then continues to use yarn as a metaphor which is woven into a poem.