Hei,

I wouldn't have ever thought the Ior rune could have anything to do with the Old Norse word "Jór".
Hestr, hross and jór all mean horse in O.N... but in slightly different ways.
Hestr is a good word for horse, but it works like "maðr"... although it does refer to the male, it can generally be used as meaning all of that kind. So as maðr means "man", it can be used for "person", and as hestr means "stallion", it can be used for any horse.
Hross is the reverse, it normally refers to a female horse, but again can just be used for any horse. Jór is just the same as Hestr, except it's a more old-fashioned term, normally something sounding too poetic for everyday use, but certainly used in poetry for effect. The word "Jór" derives from the same word that the Old English word "eoh" derives from. The Old English "eoh" is the name of the rune that makes the sound /e/.
And in usage, my good friend's name is Jósteinn which means "horse-stone". The great city of York in England used to be called Jórvik in the Old Norse language, which meant "The bay of horses". So Jór is definitely a popular word. :)

But it seems that the "Ior" rune in Anglo-Saxon fuþorc developed from the Elder Fuþark *Jéran, alongside the more common Gér rune of the A.S.Fuþorc. Ior seems to be an alternative to Gér, so whilst it's name would have come from Jéran meaning "year"... since its separate development, the name Ior had since been attributed to the word for a type of river-fish, which some suggest could mean "eel".

The names Jǫrmungandr and Miðgarðsormr are both Old Norse... but Miðgarðsormr seems to be more a nickname. Meaning the "Serpent of Miðgarðr"... Jǫrmungandr is admittedly, also a nickname, but since creatures like these have no real names of their own you must resort to nicknames anyway... but at least it is the name more commonly used... Jǫrmun means "huge" or "vast" and "gandr" is a magic staff... so here he is likened to being a huge magical creature, and very long like a staff. An Anglicisation of Norse words involves adapting the spelling to account for the English alphabet not having accents or archaic characters like eth and thorn. So Miðgarðsormr can be anglicised to Midgardsorm, and Jǫrmungandr can be anglicised to Jormungand. But there are variations on these for personal preference. :) However, the usage of the rune Ior predates significant contact with the Norse folk, and there is no evidence for the Anglo-Saxons having a world serpent in their mythology (in spite of how likely it may be), so Ior meaning eel, and sounding like the first syllable of Jǫrmungandr seems to be a coincidence I'm afraid. Nice thought though. :)
Heathen regards,

-Sveinn Fjǫlnisson