Uninterantional Icelandic names for the magnitudes of thousand.

In the Western world, specific names for larger numbers did not come into common use until quite recently. The Ancient Greeks used a system based on the myriad, that is 10.000; and their largest named number was a myriad myriad, or "one hundred million". The Romans, who were less interested in theoretical issues, expressed 1,000,000 as decies centena milia, that is, "ten hundred thousand". But it was only in the 13th century that the (originally French) word million was introduced. Until then, the name of the largest number in Latin that was uncompounded was mille (1000).
In the Icelandic as well in other Germanic languages, the possibility exists to create alternatives for the names of the magnitudes of thousand based on the Greek myriad instead on the Latin mille.
In Icelandic the ending -und in þúsund (1000, -and in English, -und in German) derives from the word hundred: þús-hund-rað (þús, thous-, taus- means 'a lot', and rað, red, -ert (*raþ, number).
In a parallel universe, history with regard to the lexical development of names of large numbers could have been different. Imagine a 15th century English mathematician developping the first magnitude of thousand, a million, by using the Graeco-English myriad (10.000) instead of the Latin mille and letting it end on -and (= hundred) as in thousand (thous-hund-red): myriand, which is exactly 1000.000. It really is possible to create an alternative term for million from the Greek myriad when developping it on a Greek or Anglos-Saxon morphological substrate. All right, first part of the problem solved. We have a solid name for the first magnitude of thousand, now the only thing we have to do to develop the other large numbers is applying the same method that was used in the creation of the existing names of multitudes of thousand: by cutting the initial character of the name of the first magnitude of thousand (as was done with the 'm' in million: billion, trillion) and replace it by the first characters of the Greek or Latin maybe even the native English names of cardinal numbers. Consider this model:

million (myriand)
billion (biryand or dyriand)
trillion (tririand)
quadrillion (tetriand, tesseriand or quadriand)
quintillion (pentriand or quintriand)
sextillion (hexriand or sexriand)
septillion (heptriand or septriand)
octillion (octriand)
nonillion (novriand)
decillion (dekriand)
undecillion (undekriand)

This scenario isn't unthinkable. Imagine if the creators of the large numerals in the 15th centrury had chosen the Greek largest single-name numeral myriad instead of the ten times smaller Latin counterpart mille to base their names upon. These words look weird at a first glance's sight, but so would be the existing ones, if we weren't used to them.

This model of creating alternatives for the names of the multitudes of thousand do actually happen to work in Icelandic because of the fact that there is lexical artefact in the old language that is similar in sound and meaning: mýr (a great many). This is what the Íslensk orðsifjabók (Icelandic etymological dictionary) has on this word:

mýr: 'mergð, grúi'. vafaorð, e.t.v. s.o. og myr (s.þ.) en ý tilkomið vegna þess að lærðir menn tengdu það við lat. mýrias, gr. mýrias (ef. mýriados) 'tíu þúsund' af mýrios (ótölulegur).

The Icelandic model goes as follows (and I would like to express my thanks to an Icelandic friend of mine who has helped me adjusting these construction)

mýrjund (million, myriand), tvírjund (billion, duriand), þrírjund (trillion, tririand), fjórjund (quadrillion, tetriand, tessyriand), fimmjund (quintillion, pentyriand, pentriand), sexjund (sixtillion, hexyriand, hexriand), seyjund (septillion, heptriand, heptyriand), áttjund (octillion, octyriand octriand), nírjund (nonillion, nonyriand, niniand), tírjund (decillion, dekriand), ellefjund (undecillion, undekriand), tólfjund (dudecillion, dudekriand).

*mýrjungur, mýrjungs- is the equivalent of the suffix myrio-, which means 1/10,000th
*mýrjund- could serve as the equivalent of mega- in mýrjundlest- (megatonne), mýrjundlát (megadeath), mýrjundrið (megahertz), mýrjundstafur (megabyte, but also gagnalest, a data-tonne), as well "mega" used in youth culture to express something fabulous. The same can be done for the other prefixes of the metric system: giga- (tvírjund-), tera (þrírjund), peta (fjórjund-), exa (fimmjund-); examples: terabyte: þrírjundstafur, þrírjundstafa-
*myrjöld means 'a million years', names for larger units of time can be formed by replacing -und by -öld: tvírjöld (a billion years, a gigayear), þrírjund (a trillion years): Jörðin er talin hafa myndast fyrir um 4,55 tvírjalda.

The term mýrjund for million and mýrjöld for "a million years" or a "megayear" could have been constructed at any point in Icelandic literary history. The Greek word has been around for thousands of years and could, in theory, have been borrowed even by medieval Icelandic scribes to construct a neologism like mýrjund at any time in Icelandic literary history. As for the other construction miklund (another possible construction, based upon the international metric prefix designating a million units: mega- (from Greek megas, the etymological cognate of Icelandic mikill) there's no lexical artifact, no historical basis in Icelandic literature to give this construction nay validity. The fact that this prefix 'mega' has been confirmed only as late as 1960 as a designation of " a million units" makes it unlikely for the 19th centrury hyperpurists and further down the road the Icelandic medieval scribes (if they would ever have experienced the urge to do so) to have come up with constructions like miklund.