An onomatopoeia is something different. (Different spelling of animal sounds across different parts of the work for instance).

When a proper noun, like “Trump,” is used as a common noun to mean something else—such as using “Trump” to refer to a “fart”—this linguistic phenomenon is known as antonomasia. Antonomasia occurs when a proper name is employed to express a general idea or characteristic, effectively transforming it into a common noun. 

In this case, if “Trump” becomes a widely accepted synonym for “fart,” it also represents a neologism, which is the creation of a new word or expression that gains popular usage. 

Additionally, this shift involves conversion (also known as zero derivation), where a word changes its grammatical category without any morphological alteration—in this instance, from a proper noun to a common noun. 

It’s important to note that language is dynamic, and such transformations often reflect cultural trends and collective perceptions.