Dragonspawn, also known as Spawn of Tiamat', are a diverse family of monsters in the Dungeons & Dragons multiverse that were created through the actions of the evil draconic deity known as Tiamat. Long story short, Tiamat occasionally causes everything from a single Chromatic Dragon egg to a whole clutch to mutate and spawn bizarre dragon-like creatures; warped parodies of their draconic progenitor. The Chromatic Dragons are "encouraged" to see this as a blessing by Tiamat, as dragonspawn exist to serve their goddess by directly seeking out and battling Bahamut and his agents - in 3e fluff, they were a direct response to Bahamut's creation of the Dragonborn. In Eberron, they exist because of Tiamat's anger with the way most Chromatics have fallen into simply sitting around and studying the Draconic Prophecy instead of going out and conquering the world.
Unlike Chromatic Dragons, Dragonspawn are highly sociable creatures and readily congregate with each other or with evil humanoids whom they find compatible - hobgoblins in particular like to work with them.
The dragonspawn debuted in the Monster Manual IV for Dungeons & Dragons 3rd Edition (except for the Greenspawn Zealot, who showed up in the MMV), and were then updated to Dungeons & Dragons 4th Edition in the Chromatic Draconomicon. They feature as a major sub-plot in Red Hand of Doom.