The plural of hippopotamus is “hippopotamuses,” though in many contexts, the Latin plural form of “hippopotami” can also be used. While both plural forms are relatively acceptable, especially when used in informal contexts, many dictionaries list “hippopotamuses” as the first entry for the plural of hippopotamus. Despite this, however, many people feel that “hippopotamuses” seems silly, both visually and when said aloud, and often reminds people of the over-pronunciation of the word in the song I Want a Hippopotamus for Christmas. This causes many people to use the other form of “hippopotami” instead, though the increased usage of “hippo” has also lead to the use of “hippos” in many instances.
Since there are essentially two correct words that can be used as the plural, there can be a great deal of confusion over which word someone should use. If someone is trying to pluralize the word “hippopotamus” in a formal context, such as a school paper or for a work report, then the word “hippopotamuses” should typically be used. This is considered the correct plural in English and avoids any of the issues that may arise from using the Latin form. Despite the fact that this may seem lengthy or somewhat silly in appearance, it is correct English and should be accepted by any teacher or employer.
The word “hippopotami” is also correct as the plural of hippopotamus, however. Since this uses the Latin plural form, it may be considered somewhat informal or inappropriate in some settings in which the English plural may be preferred. This depends a great deal on the individual preferences of those who may be reading or hearing the words, however, so people looking to use the plural of hippopotamus in a school paper or report should consult their teacher to ensure proper usage for that setting.
Since the word hippopotamus has been shortened to “hippo” in many settings, the use of the plural word “hippos” can also avoid the entire issue regarding the plural of hippopotamus. This shortening is often seen as a very informal word, however, so it should usually be avoided in contexts in which formality is needed, such as scholarly or business writings. When spoken aloud, however, the word “hippopotamuses” can become laborious to say, and so “hippos” is often acceptable in that context. Like many animals, a gathering of hippopotamuses has a particular name and such a group is referred to as a “bloat” of hippopotamuses.