Umbreon was originally intended to be a Poison-type Eeveelution for gen 1.
As we know, there were 190 missing pokemon (
http://www.smogon.com/forums/threads/there-were-190-pok%C3%A9mon.78267/). Some were saved for later for whatever reason.
Take this entry from Gold, Stadium 2, LG, and HG:
>When agitated, this Pokémon protects itself by spraying poisonous sweat from its pores. However, Umbreon can only learn 1 poison move (toxic), and not only is it by TM, it's a near-universal TM like protect. Why make these references to poison with nothing else to support it? It is because this dex entry was a preliminary entry created for Umbreon had it been in generation 1.
To an extent, it tail looks like the end of a scorpion's stinger. Its yellow rings, associated with the moon, were originally meant to reflect warning coloration on many poisonous animals. I think it was originally meant to evolve with a moon stone, so the moon associations were also intentional.
However, as eevee was cut from gen 1 and then reused for gen 2, they approached a problem: Gen 2 added dark and steel. they added steel to magnemite, but didn't add dark to any gen 1 pokemon. Darkness->night->moon, and umbreon already had moon inspiration, so they changed the type to dark. espeon was added as a counterpart, and along with the new Sun Stone, they had perfect new evolutions to utilize all of the stones. However, as they later needed an example to show off their new day/night mechanics (and an excuse to explain why players couldn't get an umbreon in generation 1), they changed its evo method to evolution at night/day. when they needed to make it a little more difficult (so players getting the eevee at goldenrod couldn't just evolve it right away), they added the friendship requirement to show off another new mechanic. From this point, eeveelutions became new pokemon to show off changing mechanics, such as glaceon/leafeon (location-based evo) and sylveon (pokemon amie/affection).
