Southern Myanmar and western and southern Thailand, and recently confirmed northern Peninsular Malaysia; reports from farther southern (to Sumatra) unconfirmed.
 
Population
Estimated population is 2,500 - 9,999 (2010) and decreasing.
Plain-pouched Hornbill (Rhyticeros subruficollis) [XC682651]
by Thet Zaw Naing from Tanintharyi (near Chaung-nauk-pyan), Myeik, Tanintharyi Region, Myanmar (flight call)
Plain-pouched Hornbill (Rhyticeros subruficollis) [XC48294]
by David Boyle from Temengor, Perak., Malaysia (flight call)
Subspecies
Sometimes thought to form a superspecies with Blyth's Hornbill (Rhyticeros plicatus). Has in past been considered conspecific with Blyth's Hornbill (Rhyticeros plicatus) or with Wreathed Hornbill (Rhyticeros undulatus), but differs from both in size, bill morphology and bare-part colours.
No subspecies.
Similar Species
Wreathed Hornbill (Rhyticeros undulatus) which has a larger, longer, corrugated bill, less peaked casque with fewer dark ridges, and blackish streak on gular pouch.
References
See References.