The Ancients were mystical beings in the Skylanders series who created everything in Skylands, as well as the Core of Light. It was said that the Ancients "were wise beyond imagination, but lazy beyond measure", a quote from the Book of Paper. The book further notes the Ancients designed amazing technology and built many incredible machines such as the Rift Engines, but long naps often took priority. They claimed the naps were a necessary part of the creative process and that personal hygiene was a mere distraction.
The benevolent Ancients created everything in Skylands using Mind Magic from their Mysterious Ancient Place, but sealed the power away to prevent it from falling into the wrong hands. However, one of the Ancients stood against his colleagues and began using Mind Magic to brainwash all of Skylands, transforming the inhabitants into mindless minions. Thanks to the magic of Dragons, who were immune to mind control, Brain's control was broken, and his plan was stopped, allowing the good Ancients to imprison their former colleague in a jar, where he would remain for thousands of years.
In their haste to explore new worlds and spread the light of Skylands, the Ancients afterwards created the Dark Rift Engine. This inadvertently released The Darkness, who proceeded to wreck havoc across Skylands with legions of evil. The Skylanders of old held back the minions of The Darkness until the Core of Light, one of the Ancients' creations, was discovered in Ancient's Peak, which successfully repelled the Darkness to the dark corners of Skylands.
Though Brain is shown to be unreliable, the Cradle of Creation implies that the Ancients had the form of exposed brain creatures. One of the many fossils or statues found in the Cradle of Creation.
The back of one of the orbs inside the Cradle of Creation.
Spyro mentions that the Ancients created the power of Mind Magic, and used it to create all of Skylands.
They are mentioned in Blastermind's backstory as having used Psionic Power Crystals that amplified their thoughts.
Brain's appearance might be the same as all other Ancients, because of the paintings, fossils and the mysterious jars in the Cradle of Creation; however, due to their shapeshifting powers, it may be only one of many forms.
If their default appearance is that of exposed brain creatures, it is possible many met their fate around the Cradle as many rocks contain what appears to be fossilized remains, not counting the ones found inside the mountain, frozen in place inside orbs.