Chicken in the Cold Plains
(Note chicken just above and to the left of the waypoint.)
I and a friend set up a Open game for the purpose of sorting some problem or other she was having with trading a rare amulet to another character. It kept disappearing, and her PC kept crashing, so my job in all this was simply to hold the game open for her various chars to pop in and out, occasionally actually storing whichever item was being tested in my inventory.
I didn't want to get too engrossed in questing or in fact doing anything dangerous while waiting for one or the other of her chars, so after taking Tankgrrl, who was level 24 at the time, on a quick run through the Catacombs to see if Andariel would drop any interesting gems, I was just bumming around the Rogue Encampment when I noticed I could push a chicken out the gate and into the Blood Moor.
Shoving this chicken around required me to use absolutely none of the skills useful in playing Diablo. Or indeed in any other endeavour. This was frankly such a worthless exercise and complete waste of time, I became intrigued.
With faithful Thadar to make sure the monsters didn't stick around long enough to scare my chicken, I focused my efforts entirely on following the dirt path through the Blood Moor and then the Stony Field (Flavia was a bit of a nuisance at the entry—she kept getting in the chicken's way), and past the Cairn Stones ("Rakanishu!" "Yeah, yeah…" [WHACK] "Rakanishu's dead, ya little punk.") and well into the Cold Plains. At that point I found a gem shrine and lost the chicken when I portaled back to camp for a topaz, but in any case, I'm pretty sure I couldn't have gotten the chicken to go down into the Underground Passage anyway.