So, stuff like this?
Jot, Quasit - Sunless Citadel (a run of the mill quasit, I believe)
Idalla, Succubus - Forge of Fury (a run of the mill succubus, I believe)
The Cuckoo, Vrock Bard - Standing Stone (presumably referenced again in Heart of Nightfang Spire)
Rhunad, Vrock - Heart of Nightfang Spire (a run of the mill vrock)
Ammet, Half-Dragon Balor - Bastion of Broken Souls (the original was advanced to 19 HD. It is unclear if it should be advanced in 3.5 rules, but presumably not; was bound as Ashardalon's heart; Has a three-strand artifact whip named Helltongue; called Eater of Souls)
(Ashardalon, Half-Fiend Great Red Wyrm - Bastion of Broken Souls; bound Ammet as his heart)
The Cathezar, Half-Chain Devil/Half-Marilith - Bastion of Broken Souls (originally a servant of Demogorgon/Aameul; depending on events, she may be either dead or serve Ammet now)
Buphalthus, Balor - Bastion of Broken Souls (has 2/day Implosion SLA instead of fire storm)
Tamion, Balor - Bastion of Broken Souls (Wields a +3 vorpal unholy greatsword; also has a cloak of resistance +4 and a belt of giant strength +6)
I didn't doublecheck most stats, so some of the run-of-the-mill creatures might not be.