Changelings have the size and physical anatomy similar to Ponies, though do hold some notable similarities to invertebrates. Changelings all have insect-like wings, sabre teeth, hard armor on their backs resembling beetle armor, and manes and tails resembling fins rather than fur or scales. Male and female Changelings are indistinguishable from one another, though other variants had been known to exist, mostly an alteration in color on back armor and eyes. Highest ranking queens are the largest ones in a hive of Changelings, about as tall as an Alicorn.
Changeling blood is green, unlike most other animals.
Changelings, despite their thinly and seemingly fragile frame, are capable of surviving otherwise fatal injuries thanks to their magical abilities. A Changeling is capable of regrowing lost limbs, and capable of transforming themselves into any living thing they know of. They're notorious for feeding off of others' love as their main source of food. While it isn't exactly said if they eat anything else, Chrysalis considers it sick to actually eat flesh off of another pony, hinting that they're not carnivorous.
Said by Thorax, a changeling queen can have numerous descendants, often an entire hive being the offspring to one queen.
In the case of Chrysalis's hive, Changelings fall under a very strict hive mind system, in which the queen of the hive is in complete control over every other Changeling in said hive. Any Changeling that leaves said hive is considered a traitor and brutally punished.
The first Changeling to officially appear is at the end of S07Ep12: Dragon Lord, being a Changeling disguised as a dragon for the Gauntlet of Fire to steal the bloodstone and destroy the Bloodstone Scepter. The intention was to make sure there would be no Dragon Lord for Equestria, as no scepter means no winner for the competition.
The full horde of Changelings first appear in "Rogue Ruby" as the hive discovered a group of Gems and Ponies entering the Badlands. The Changelings proceeded to capture Ruby and Steven Universe, replacing them with Thorax and Pharynx once they were secured. The Changelings during this time were creating a Warp Pad to enter Earth to better arm themselves and reclaim Equestria for themselves to even survive, but their plan failed thanks to the Tindalos attacking them, and forcing them into a fight between the Crystal Gems, Mane Six, Pillars of Equestria, and the Windigo.