While they are normally of similar height to humans and other humanoids, the episode in which they are most prominent, "A Tale of Two Santas," a subpopulation are depicted as diminutive due to malnutrition. The subpopulation in question were living near Neptune's North Pole under the rule of Robot Santa, where they work as "elf" toymakers. Due to Santa's nature, they live in a ghetto-like society, in squalor and extreme poverty, foraging for scraps to survive. It is because of this low quality of life, and Santa's lack of caregiving, that the Neptunians' growth is stunted.
The Neptunians living near the North Pole are hinted to be predominantly homosexual; this was confirmed in the audio commentary for the episode. Elzar, however, seems to be heterosexual, so it is likely the high incidence of homosexuality in the North Pole subpopulation is not universal.
Other than Robot Santa's elven slaves, the only notable Neptunian is the chef Elzar. Other Neptunians appear as background characters, such as Beck's drummer in "Bendin' in the Wind," a court security officer in "A Taste of Freedom," Heather the spa worker in "Teenage Mutant Leela's Hurdles," two alien pornographic stars using "Human Horn" in the episode "Spanish Fry," the representative for the political party "People for the Ethical Treatment of Humans" in "A Head in the Polls," and a market owner in "My Three Suns" (where it is shown there is an area of New New York called Little Neptune).