Horatio Humboldt was an English explorer in Grand Theft Auto IV. He is based on the real life person Henry Hudson. Humboldt was hired by the Dutch to find a new place to sell weed. On September 4, 1609, he steered his ship into the mouth of a river coincidentally named the Humboldt by the local natives, based on Humboldt's writings. He is mentioned on CNT's show A History of Liberty City.
Humboldt's name is presumably based on Alexander von Humboldt, a German naturalist and explorer after whom many geographical features were named.
Humboldt claims that the natives have already named the river he finds the "Humboldt", although this might be a lie to justify his naming of the river after himself.