Battle Moveset

A Good Moveset for Jolteon would have to be this:

Attacks:

Thunderbolt
Hidden Power Ice OR Grass
Substitute
Baton Pass OR Thunderwave

Items Attached:

Petaya Berry
LeftOvers

Preferred Nature:

Impish

Strategy Using Jolteon

With a base speed of 130 (and a potential 394 final speed stat), Jolteon can and will outrun most pokemon. It should be able to outrun Aerodactyl (assuming it doesn't have a +speed nature and 252 speed EV's), it should be able to KO it with ease (Thunderbolt) before getting hit by an Earthquake.
Since the main type of Pokemon Jolteon has to be careful of are Ground types or Pokemon that can learn Earthquake, Jolteon should at least have Hidden Power Ice. This takes care of most ground types (apart from pokemon like Swampert, hence the reason HP Grass was included) and Salamence (which is usually carrying Earthquake)
For Hidden Power Ice, I suggest these IV placements: 31/02/30/31/31/31 and for Hidden Power grass: 31/03/30/31/30/31.
Jolteon can set up a fast substitute to protect against Status damage or attack damage. Prediction helps too, especially if you know your opponent plans to use an Electric attack against one of your current pokemon, then you can switch to Jolteon and it will regain 1/4 of its HP (making up for the loss in a substitute).
Thunderwave and Baton pass are pretty obvious choices (stall with Thunderwave, Baton pass in case of Arena traps/Mean look pokemon), so use either one if you know for sure what your opponent intends to use in his team.

EV Corner:

Max out the EVs in Speed and Special Attack for Jolteon with any remaining in HP

Strategy Against Jolteon

Jolteon is one of the four (three not counting Mewtwo) Base 130 Speed Pokémon, Base 130 being the fourth highest Base Speed in the game. Jolteon has a good Special Attack stat to go with it alongside a respectable Special Defense.
- Firstly, don't try to catch it with a status unless you are using something faster (such as Electrode, Electrode with Thunder Wave could spell an end to Jolteon - just so you know), since most Jolteons will just set up a Sub on the first turn and block your status.
- Also, don't expect to keep Jolteon trapped via Dugtrio's (or Trapinch's) Arena Trap. Although not all Jolteons carry it, some do have Baton Pass, which will allow them to escape their situation. Same goes for using Mean Look on it.
- Prediction is your friend when you know your opponent has a Jolteon. If he has something weak to Electric attacks and knows you have an Electric attack, he'll more than likely switch to Jolteon who will gain back a ¼ of its lost HP (the equivalent of what it loses with Sub).
- "Always strikes first" style moves are always a good thing against something as fast (and as weak defensively) as Jolteon, since you'll definitely score some decent damage (hopefully by surprise).
- If you want to take down Jolteon your best bet is surprise. If you can catch it with EQ while it switches in you will most likely have a KO. Jolteon will simply Sub before you attack and since you (probably) can't Taunt before it uses it, you are forced to whittle down its HP by forcing it to re-Sub until it is forced to switch or take the hit.

Contest Moveset

A good Contest Moveset for Jolteon would have to be this for the Cool Contest best with Lonely, Adament, Naughty or Brave Nature:

Attacks:

Rain Dance
Thunderbolt
Thunderwave
Hyper Beam

Items Attached:

Red Scarf

Strategy Using Jolteon

If you do the attacks in this following order you should have very few problems; 1st - Rain dance, 2nd - Thunderbolt, 3rd - Rain dance, 4th - Thunderbolt, 5th - Hyper beam/Thunderwave

Locations in Games

Ruby/Sapphire/Emerald Trade from FR/LG Colosseum Trade from FR/LG Fire Red/Leaf Green Evolve Eevee

Animé Appearences

Jolteon has had a few Animé Appearences. First it was owned by one of the Eevee brothers. After that, Ash battled one in the Pokemon League Entrance Exam. After that one of the Kimono Sisters in Ecruteak City had one. The Rest are Cameos Episode 040: The Battling Eevee Bros! Episode 056: The Ultimate Test! Episode 103: Get Along Little Pokémon! Episode 185: Trouble's Brewing! Episode 228: Espeon Not Included!