Heterochthonous is an uncommon word that describes something or someone that is not originally from a particular place. Synonyms for this word can include terms like exotic, foreign, alien, non-indigenous, or non-native. These words can be used to describe plants, animals, people, or even ideas that have come from outside of a particular region or culture. Other synonyms could include migrant, immigrant, newcomer, or intruder, depending on the context of the situation. While heterochthonous may not be a term that is commonly used in everyday conversation, there are plenty of different words that can be used to describe this concept.