As a character, DiscordDiscord's original design came straight from Lauren Faust herself, which in turn took inspiration from the character Q in Star Trek (and this should explain why they got De Lancie himself to voice him, see here for further details).
Jokes aside, it is pretty clear that a simple "dragon horse" wasn't what she had in mind. As you can see in this concept art kindly taken from her DeviantArt account, Discord was planned as having multiple animals traits from the start.
We first hear the term from Cheerilee in "The Return of Harmony Part 1 - S2E1":
Cheerilee: This creature is called a draconequus. He has the head of a pony and a body made up of all sorts of things. What do you suppose that represents?
Apple Bloom: Confusion!
Sweetie Belle: Evil!
Scootaloo: Chaos!
[...cut...]
Cheerilee: Actually, in a way, you're all right. This statue represents discord, which means a lack of harmony between ponies.
- first she describes the feature of a draconequus as "made up of all sorts of things" - not just Discord
- yet later she also describes a draconequus as a representation of "disharmony"
It is also worth noticing that Scootaloo answer was "Chaos", yet based on Cheerilee answer disharmony and chaos are two separate things.
It is also worth noting that Scootaloo's answer was "Chaos", yet based on Cheerilee's answer disharmony and chaos are two separate things.
As you can see when Discord was first introduced, he was described as the draconequus embodiment of "disharmony". This is reinforced by the fact that the scene shows the seal on Discord breaking as ponies getsget in an argument near the statue. This scene is telling us that Discord is empowered by the three Cutie Mark CrusadersCrusaders' fight.
Later, in the same episode, Celestia again calls Discord a creature of disharmony:
It is only in the next episode than Discord defines himself as a creature of Chaos.
Twilight Sparkle: Stop it, Discord. You're not playing fair.
Discord: I'm not playing fair? Perhaps we haven't met. I'm Discord, spirit of chaos and disharmony.
At this point, some small incongruousnessincongruities start to come up.
Are all draconequus really an "embodiment of disharmonydisharmony"?"
We never see another draconequus in the series - the only one is Discord. Therefore, we can't really argue if "other" draconequus are like him simple, simply because we don't know any.
Yet it is worth noticing that Accord - DiscordDiscord's alter-ego in "My Little Pony: Friendship is Magic Issue #48-50 story arc Chaos Theory" - is both a creature of pure Order and still a draconequus. Therefore, it doesn't seem likely the two things are directly tied.
Are all "embodiment"embodiments of disharmony" draconequus?"
Princess Eris, first introduced in "My Little Pony: Nightmare Knights" as Discord's cousin in the goddess of Chaos of an alternate universe (yes, don't overthink how this seems oddly similar to a certain "Cat of Destruction" and his brother...) is... not a draconequus as we know and never uses the word. If anything she is more close to a red parrot, or to be more precise a Sarimanok (which does indeed mean "assorted bird" so there is a strong possibility someone took that as "bird made up of different parts" - a parallel to the draconequus).
This proves that not every "creature of Chaos and Disharmony" has to take the shape of a draconequus. Some take a step further and claims that this is evidence of DiscordDiscord's current appearance being something he choosechose.
At first, DiscordDiscord's main nature is that of "Disharmony" - lack of friendship, the main show theme. We can se it in his name - Discord comes from "Discordia", the Latin name of Eris, the goddess of discord. And as you just saw, Discord's cousin is called Eris.
In ishis first appearance we see Discord's Disharmony side. He acts as a malevolent creature that takes power from discord and strife. He enjoys subverting the mane sixsix's nature and turnturns them into their oppositeopposites - what some fans call the "Element of Disharmony". He is actively evil and wants to do harm.
Later, this startstarts to fade as Discord's second nature become more evident: Chaos.
Once again Equestria seems to parallel Origin's Ultima. Chaos doesn't seems to be inherently evil but just a force of nature closely related to entropy. "Chaotic" Discord only want to have fun and subvert things, but once reformed and becoming friend with Fluttershy he is perfectly able of being chaotic while also being a friend that likes to laugh withwith others, not atat others. In this regard it should also be noticed that Accord, in his misguided attempts, seems to instead see (forcedforce) Harmony as a way to reach pure Order. This should not surprise you, since Accord in that story acts as a Bane of Order, and thus is taking the nature of Order to its extreme (this is similar to Ei in Genshin Impact trying to create immutable Eternity by removing peoplepeople's ambitions).
At this point we can conclude one thing: Discord is a creature of Chaos that was misguided in thinking he needed Disharmony but does need to.
Notice how "acting normal" can harm Discord, yet being friendly with Fluttershy causecauses him no harm. It is unclear how things changed in development (DiscordDiscord's name seems to indicate the staff initially intended chaos and disharmony to be the same thing) but at the end Discord is just a creature of Chaos.
So, to answer your question...
Discord is a creature of Chaos and Entropy that taketakes a form that represent that Chaos. During his life he had a few friends and become a pawn of Disharmony, thinking that Chaos should subvert friendship. Later, a yellow pony showed him that was not the case and that he could just be a (sometime annoying) prankster that loves been nonsensical and chaotic.