Chiropterans are vampiric hematophagous bat-like creatures that live by feeding on human blood.
What We've Seen
The chiropterans shown were created from the blood of a queen. In the anime, Diva's blood is used to create most of them. However, unlike the process of creating chevaliers, creating chiropterans takes an indirect injection of queen's blood. The blood is first taken from the queen. This is either injected into live human bodies or manipulated into delta particles which are used in all cinq fleches products. The consumption of the products leads to gradual transformation into a chiropteran. The process can be sped up with using enhancement like Diva's voice. Partial transformation can take place as well, e.g. in case of Mui. Sometimes, receiving scar injury from chiropterans might also lead to partial transformation, e.g. in case of George (however, it's unknown if the infection was due to medical treatment given to him after he was kidnapped from Yanbaru facility).
Chiropterans Outside of The Lab
Since such characters have not been seen in anything release, the lab made chiropterans have been created and coined as the regular chiropterans. However, fans and even members of the production have had questions to others of their kind who aren't Queens. One such character who, in all actuality, has a chance at being a true "standard" chiropteran is Nathan. However, there are many theories about who and what he may actually be.
If it were to be revealed that Nathan were simply a chiropteran from an older generation or time with a higher population that leaves many questions. Regardless we would have confirmation that, in such a theory, natural chiropteran do not die from the blood of Saya and furthermore, theoretically, any queen. In addition, it would prove that they can survive, regenerate and still give their blood to the alternate queen. Making it questionable for their own use a cure to the crystallization of their blood's mix.
Still, there is the possibility that this is simply a description to a higher class, royal sibling of queens, or a set of queen's mother's chevalier. As we do not know what happens in either scenario, nor do we know Nathan's exact history, origin, and relation to the other chiropterans is untold.
Despite all the unconfirmed information it is possible that others outside of twin lineage do exist. Although they are not created through the blood of a queen, like lab chiropterans and chevaliers are, so they would presumably have the benefit over the others to not have any kind of immediate master or siring commander. Perhaps if they do exist, they avoid showing until after the current generation's fight, as to not be put in the midst of it, or ever to avoid a Queen's ability to compel.
They generally possess extraordinary speed, strength, and supernatural healing abilities. They are also capable of contorting and transforming various body parts, in order to fly, or have longer limbs. Higher level chiropterans have the ability to disguise themselves as ordinary humans.
Chiropterans can heal almost instantly from most wounds. Thus, bullets are non-lethal and only capable of slowing them down. Explosives as well prove to be ineffective and/or non-lethal. However, if buried or sealed inside stone they will go into hibernation until freed
Process of Elimination
Ironically, the blood of queen is also the primary method of killing them. When the blood of another queen is introduced into a chiropteran's bloodstream, it starts a crystallizing chain-reaction that destroys them. Thus, chiropterans created from Diva's blood can be killed by Saya's blood, and vice versa. Another way to eliminate a chiropteran is by the use of decapitation even though it is believed not to be a viable method for humans but it does give a much higher chance of success than the use of bullets. Blood loss is another method to kill these creatures as without blood the Chiropteran's strength, speed and healing capabilities are greatly diminished, allowing bullets to be more effective than previously possible.
Chiropterans seem to be very inspired by Vampires, similar hematophagous humanoid creatures of European folklore.