The appearance of Homunculus is identical to Shinigami, besides their clothing. Their clothing ranges from royal robes to rags that only the poorest of the poor wear. Unlike any other species of souls, they possess a body part taken from their respective form, consiting of various animal parts. Other than that, they look like normal Shinigami. When in their "Homunculus Mode,", they take on their more animalistic form. An example of this is when Tsume transformed into a crocodile after being put under extreme stress. This form is mostly brought out unintentionally while under extreme emotions. However, some may learn to master this side of themselves and bring it out when they see fit.
Depending on if a Homunculus is natural or artificial, they will have different abilities. The only difference is that a natural Homunculus are in possession of a zanpakutō.
Hand-to-Hand Combat: Due to only natural Homunculus having a weapon, most Homonculus rely heavily on martial arts. They have been known to be extremely adept at using it. Many use their "gifts," as they have come to call their traits, to add to their power. Combined with speed, they can easily defeat even strong opponents in a matter of moves.
Rapiditas: (Latin for "Zip"): Known to be the equivalent to Shunpō or Sonído, Rapiditas allows Homunculus to cover great distances in a very short time span. While most Homunculus cannot even begin to master it, a select few have learned to master it to a level that they can keep up with the likes of Shunpō Masters, such as Byakuya Kuchiki.
Validus: (Latin for "Strong"): The equivalent of an Arrancar's Hierro, Validus gives a Homunculus the ability to harden their spiritual pressure to the point that it can protect them from damage. Unlike an Arrancar's Hierro, the Validus ability isn't incredibly strong, but just strong enough to protect them from minor injuries. Most Homunculus consider this useless and rarely take the time to learn it.
Exceritus (Latin for Army): A mark of a natural born Homunculus, much like Kuwa, they are able to create an group of lesser Homunculi whose soulless bodies are stored in an separate pocket dimension. To grant them life, these homunculi are fueled by the summoner's own energy. These mannequin-like Homunculi are white and red, skeletally thin, hairless humanoids with one eye in their foreheads. They lack intelligence and appear only to follow base instincts toward feeding and destruction, listening to the owner to destroy anything in their surroundings. Their main attack is to try to eat their victim, since they lack the thought processes to do anything else. They do not regenerate from damage, but are able to keep functioning even if their heads are grievously damaged, making them similar to zombies in a way.
After being put under extreme emotions, or by sheer will, a Homunculus can manifest their "Homunculus Mode," which allows them to become their more animalistic or mutated humanoid form. When in this form, they can use their "gifts" to their full effect, such as using a tail to grab someone, or perhaps throwing them a distance.