Changeling drones are equivalent to an adult pony. Their androgynous bodies (namely, their lack of genitalia and nipples) allow them to transform into humans of nearly any frame. The only outstanding difference between each changeling is their level of muscle mass; each changeling is of a different musculature. A changeling that looks akin to a slender woman could appear as a stallion, but his loved ones would immediately notice his lack of muscle. Likewise, a changeling that looks like a strong man would make for a rather obscenely bulky woman. Imagine the surprise of a working husband that comes home to see that his wife suddenly has biceps like that of a Royal Guard.
The Queen, on the third hand, is the only changeling that has any sign of gender, as indicated by her breasts and her vagina. In the aspect of anatomy, the changeling queen is identical to a human female in almost every way. The queen, like her drones, however, has long, sharp fangs that plunge into a human's skin so she can drain its body of nutrients. She bites down on different parts to absorb different types of nourishment: if she needs protein, she tears through muscle, or bone if she wants calcium. In addition, all changelings have the same pitch black skin color, unlike humans who have a variety of shades and tints to... whatever color they view as their standard.
Only when she is birthing her heiress does another changeling with the nipples to feed a child come into the world. This one of several facets of changeling culture that has essentially alienated their kind in the minds of humans.
When the heiress is born, her body, the approximate equivalent to a human girl of five to six years in age, is completely developed in a sexual sense, and theoretically, she could birth another changeling immediately after. The only reason that she doesn't immediately start pumping out drones is the fact that her uterus is not large enough to house a changeling worth birthing. She could very well force her body to make a drone, and past young rulers have, but their ambitions are cut short when the resulting drone is born still. Sometimes it's skeleton would come out deformed, or it could have been crushed by her inexperienced vaginal muscles. It is the job of her mother to teach the young girl that she shouldn't attempt to give birth until she is ready, and to demonstrate how to do so correctly and easily. It takes years of growth for her uterine and vaginal muscles to be thick enough to facilitate the birth a changeling, drone or otherwise.
Aside from being the only changeling that can give birth, the queen and her daughter are the only ones who are 'intelligent', by human standards. The drones are technically sentient, but are completely and utterly loyal to their queen, who is in their eyes a goddess to be worshiped. If you think about it, she kind of is; she, and her special offspring, are solely responsible for the continued existence of her race, and when her drones are near or touching her, their minds become little more than extensions of her consciousness, like arms. Her connection to them creates a sort of hive mentality; they feel her emotions and want to make her happy because when she is happy, they are happy.
It is here that the human and changeling cultures divide most drastically; changeling girls do not require a male to give a genetic sample to create a drone. All of the necessary genetic coding is in her own. To begin the pregnancy, however, the changeling must go into what is essentially a kind of heat, like many wild animals do. However, because changelings are so humanoid, their way of going into heat is the same as becoming aroused, and humans as a culture frown on the idea of a child becoming sexually active.
This faux pas was born out of ignorance towards the changelings' biology. The heiress' mind is much more developed and mature at birth than that of an equivalent human, and her sexual arousal doesn't mean the same thing as the sexual arousal of a human girl of an age that would make their bodies look similar. However, the differences don't stop there. To teach her daughter how to become aroused of her own volition, the current queen generally performs certain... sex acts on the younger one's erogenous zones to kickstart her first few births. This goes on until she learns how to do the stimulation herself. However, like before, these acts don't carry the same stigma in changeling culture as they do among humans; the act of the queen fingering her daughter is merely a method of passing on the mantle of the changelings' age-old birthing process to the next queen.
Yet another reason for humanity's aversion to changeling culture is their gestation period. A human child takes approximately nine months to grow and develop inside of its mother body before it is absolutely ready to breathe and beat its heart on its own. Changeling drones take only several grueling, painful days of their mother's time to fully develop. This is owed to the much simpler anatomy of the drones; their life-support systems are far simpler than that of their human counterparts, and they do not have reproductive systems at all. The queen, however, can actively speed up the development of her fetus, as long as she maintains a state of difficult concentration. This, coupled with the magnified pain that she feels fro the increased nutrient drain that the fetal development exacts on her, makes quick birthing a rather unpopular option, in the opinions of past queens. Queens of incredibly strong will, and intestinal fortitude, have been known to be able to pump out drones every few hours, but they generally don't last long; queens in the past have died from the strain this process puts on their bodies, and the changelings count themselves lucky that there was already a successor by the time that it happened each time. Louder screams have never been heard than from mothers forcing their drones to grow faster; those that survived have compared the intensified pain as akin to "having my entire organ system being yanked out through my [the mother's] vagina".
Cultural differences such as these have made it difficult for the changeling race to interact with the modern human society. In the past, changelings got along well with humans because children were not as sheltered from the concepts of sex and arousal like they are now. While the social standards for raising children have changed with time, changeling biology has not. While Celestia and Luna were not unaware of changelings' reasons for 'fingering little kids', it was with heavy, regretful hearts that they halted trade with the changeling race out of fear for losing support from their subjects. The public could not understand that the changelings were good, innocent people, and that ignorance lead to the shunning of changelings, as a race, at the hands of the humans.