It's obviously some D2 reference but I never played a diablo game before and I've been wondering, what is this Cow level that everyone talks about?
There is no cow level. Everything you know is a lie.
http://diablo.wikia.com/wiki/The_Secret_Cow_Level
There you go.
Other memory :
Moo
Moo
*splat*
FUCKING HELL WHO KILLED THE KING
*remakes char*
Cow Level, in Diablo 2 anyway, was created when you combined a Tome of Town Portals and Wirt's Leg in the Horadric Cube
Then you'd kill lots of cows walking on 2 legs weilding polearms.
I've never heard of a pony level, but the cow level was always fun.
The cow level
Was a great place to farm gear
“Alcohol may be man's worst enemy, but the bible says love your enemy." ~Frank Sinatra
It was a blast going into the cow level with an amazon, lightning fury + perfect eth titan's revenge killed so many so fast
So there will be no COW levels. That doesn't mean there won't be x (fill in another type of animal/monster here) levels.
There ain't any cow level. Never been. Don't trust what they say, it's all a lie.
At least that's the joke
IIRC the whole thing started with a false rumor about Diablo 1 having a secret "cow level", which Blizzard then put in as an actual easter egg area in D2. This zone was extremely popular among powergamers because of being an easily exploitable level grinding area prior to being nerfed in a patch, and that may have helped reinforce the popularity of the catchphrase. They would then go on to reference the cow level (or rather the alleged inexistence of it) in other games such as the sheep level in Frozen Throne and loading screen tips in WoW. It seems from early datamining as if Diablo III in turn will have a similiar easter egg area, dubbed the "pony level" by speculators for most likely referencing the My Little Pony: Friendship is Magic cartoon series in certain spoken lines and boss names.
That's pretty much the whole deal. Just a somewhat obscure-in-origin Blizzard tradition dating back to an old community joke.