Diablo's 20th Anniversary

When people first started getting the achievement, I thought it was something you had to play in Diablo and then log into WoW and receive an item. It was better than that. There was an ingame thing to do.

There were treasure goblins all over the place, and you kill them to get toys, scrolls, a bag and other goodies. Killing them opened a portal to the Cow Level. I have never understood the reference except that it was something to do with Diablo so I had to go look it up.
Way back with Diablo 1, there were rumors that clicking repeatedly on the corpse of a specific cow would open up a secret level. Blizzard denied this (rightfully so), and made a humorous nod to the rumor by including a cheat code in their game Starcraft.
Not helping rumors is the fact that there actually was a Cow Level in Diablo II, and there have also been numerous other cow jokes and "there is no cow level" jokes hidden in Blizzard's other games. For example, it was the cheat code in StarCraft used to skip the current level. The line lives on in the hearts of long-time Blizzard fans and as an occasional loading screen tip in World of Warcraft.
- WoWWiki 

Wirt's third leg, which drops in Tanaris, is a reference to this Cow level. In D2, there is a character,Wirt (who lost his leg to Diablo) whose peg leg you can use to open the Cow level. Cow King's Hide, which is a world drop blue BOE, is a reference to the Cow King. 

Treasure Goblins reliably spawnyh in Dalaran sewers. They would also be at the end of each dungeon. They were also found all over the world too, but randomly. However, once they start doing their portal you need to kill them quick! I had a dreadful time trying to kill goblins because people had already done their achievement and couldn't be bothered doing anymore so not enough DPS on goblins and they'd run away.

Portals would spawn taking you to the Cow level where you would kill  the Cow King. Really hard to get a picture of him, this was the best one I got. He drops a Twelve String Guitar and once you add that item to your Toy collection you get the achievement!

There was also a whole bunch of items that fill up your bag which gave you extra stam, speed or armour (I never saw an int one) which still give you that slight buff. I might throw them out now - they are only piddly buffs, after all. I even got the 28 slot bag.

But what I wanted was the Town Portal toy, which needed 10 Scrolls of Town Portal. But I left it till the last day and then spent a couple of hours trying to kill goblins, doing heroic dungeons and hanging out in Dalaran Sewers. A fair number got away. Tacky came to help me and I finally managed to get my last scroll which was a relief!

Happy 20th Anniversary Diablo! And there WAS a cow level...