The manager of a large city zoo was drafting a letter to order a pair of animals. He sat at his computer and typed the following sentence: "I would like to place an order for two mongooses, to be delivered at your earliest convenience."
He stared at the screen, focusing on that odd word mongooses. Then he deleted the word and added another, so that the sentence now read: "I would like to place an order for two mongeese, to be delivered at your earliest convenience."
Again he stared at the screen, this time focusing on the new word, which seemed just as odd as the original one. Finally, he deleted the whole sentence and started all over. "Everyone knows no full-stocked zoo should be without a mongoose," he typed. "Please send us two of them."
According to Dictionary.com and the American Heritage Dictionary the plural of mongoose is mongooses which makes sense because etymologically there is no relation to the word goose. The word mongoose comes from mangus, a word from the Indic language Marathi.
Interestingly, the Merriam-Webster Dictionary does include mongeese as an accepted plural form. This is likely because of its repeated usage by people unfamiliar with the etymology of mongoose.
To learn more about what qualifies a word for dictionary inclusion please refer to the previous post on this topic.