A Chicken is an item in EarthBound and Mother 3. It can be obtained if Ness and co. have a Fresh Egg in their inventory and wait. The egg will eventually hatch into a Chick, which will then grow into a Chicken.
If used, the Chicken will flee from the character's inventory, but they can be sold for considerably more than the Fresh Egg it originally came from is worth. This can be used as a simple way to obtain money early in the game. In Earthbound, the Chicken sells for $110 while Fresh Eggs cost $12, earning $98 in profit with each egg. The Fresh Egg can be purchased at the self service stand, in Happy Happy Village. If you do not pay and you talk to the local watching the stand, he will attack you. He doesn't do much, and you could easily beat him in one hit. After that, you can buy whatever you want from the self service stand. After defeating Mr. Carpainter, the stand will go out of business due to personal reasons (Though, the true reason is that Porky stole all the food from it).
In Mother 3, a Chicken will periodically make clucking noises as long as it is in a character's inventory. Chickens also appear as NPCs at Alec's house. As an addition, near the Chimera Lab, the hot spring houses a chicken who will give you a Fresh Egg just by talking to it. If the player bathes in the hot spring with a Fresh Egg in their inventory, the egg will transform into a Hot Spring Egg. The hot spring egg heals 100 health, just slightly more than the Fresh Egg. Delicious!