In Dungeons & Dragons (and without a traditional mythological counterpart), the intellect devourer is a monster first appeared in «Eldritch Wizardry», back in 1976. In its first apparition they weren’t yet brain-like, its initial description being “frightening in itself, for they have no apparent head, being merely a ball-like body of sooty black poised upon four legs.” An intellect devourer is a chaotic and evil monster, highly malign with regard to sentient life. It dwells deep beneath the ground, and subsists on the psychic energy of its prey.
An intellect devourer resembles a walking brain protected by a crusty covering and set on bestial clawed legs. This foul aberration feeds on the intelligence of sentient creatures, taking over a victim’s body on behalf of its mind flayer masters.
In 4e’s Monster Manual 3, they also have a larval form, called ustilagors, in which they are little more than ratlike vermin that skulk in the shadows and feed on the psychic energies of larger creatures. They are said to be formed from the brain tissue of creatures killed by other intellect devourers. As adult intellect devourers («predators»), they are more akin to wolves, hunting in packs or serving more intelligent creatures as guard beasts. Those rare few that survive long enough metamorphose a second time, becoming the truly monstrous intellect gluttons, which are also called brain collectors.
Of all the terrors in the Underdark, intellect devourers are among the most feared. These hideous creatures carry the taint of the Far Realm, roaming deep tunnels and dark caverns in search of sentient prey. When it attacks, an intellect devourer consumes a foe’s psychic energy, drinking deep until only an empty husk remains.
Mind Flayers breed intellect devourers to serve as roaming hunters of the Underdark. They do so by taking the brain of a thrall and subjecting it to a horrible ritual. As it sprouts legs, the brain becomes an intelligent predator as twisted and evil as its masters.
An intellect devourer consumes a creature’s mind and memories, then turns the host body into a puppet under its control. They typically use its puppet host to lure others into the domain of the mind flayers to be enthralled or consumed.
In your campaign
When using an Intellect Devourer, be careful: not many heroes can resist its attacks properly (with the exception of the wizard), so with a few bad rolls it can turn your adventuring party into a brain puree (fortunately, the DC for such attack is below averageat 12). I use them as my Mind Flayer mastermind pets (one chained at each side of his) creating an utterly vision of danger into my players.
Intellect devourers can be found wherever aberrant creatures gather, either among their own kind or in service to more powerful masters. Mind flayers frequently keep intellect devourers as pets and sentries. The largest intellect gluttons sometimes serve as steeds for mind flayers and other cruel creatures.
You can download a 5e’s Monster Manual preview featuring the Intellect Devourer directly from Wizards of the Coast website here.