Xt'Tapalatakettle
Xt'Tapalatakettle, also known as the Olmec Head, is a large rock depicting the Olmec Indian God of War of the same name. In the episode "Blood Feud," Mr. Burns gives it to Bart.
The $32,000 gift was given in appreciation to Bart for donating some of his blood, a rare type, to save Mr. Burns's life. Homer is disappointed with the present, but Bart thinks it's cool.
Several other episodes show glimpses of the head -- usually stored in the family's basement, attic or cellar -- including "Homer Alone," "Two Bad Neighbors," "The Day the Violence Died," "Burns, Baby Burns," "Homer the Great," "When You Dish Upon a Star," and "Treehouse of Horror V."
There is also a story titled "Xt'tapalatakettle's Day" in the comic book Bart Simpson's Treehouse of Horror 5.
It has also been featured in the non-canon Simpsons Comics, where it was revealed that an Olmec priest lived in the Head, awaiting the judgement of Xt'Tapalatakettle on mankind. Lisa became envious of his bond with Snowball II, and together with Bart, tricked him into thinking that Xt'Tapalatakettle had judged mankind to be poor. Mr. Burns gave him a plane ticket home in order to hold onto his Oldest Man in Springfield Award. Incidentally, Xt'Tapalatakettle returned just after the priest left, but learnt of donuts, beer and television, leading him to deem mankind worthy.