Well there are the 'bigger' defilers. I can't remember what they are called though, just that there were pics in the old codex and people stared building them. You can also take any Imperial vehicle and chaosify it for apocalypse.
There are also some nice conversions here. You can make an upright defiler into a chaos knight titan, similar to this thread. http://z4.invisionfree.com/ironwarriors/in...topic=3051&st=0
Other chaos warmachines...
Chaos Decimator: A version of the baneblade, all I know is that its main cannon ignores cover and has really good range.
Death Wheels(my favorite): Imagine a giant wheel the height of a warhound rolling across the field on spikes the size of marines . The whole thing is gyro stabalized, demonically possessed, and on each side of the giant spinning wheel is a stationary(not spinning) hub with a battlecannon and 2 reapers. So 1 battlecannon, and 2 reapers on each side of the vehicle. Now I'm transferring its stats from epic based on what they would be in 40k. It has 4 structure points, 2 void shields, and moves 12" a turn, and has the weapons described.
I've already built death wheels for epic, I just took 40mm bases stuck together, and then put 40k bike tires(cut in half) on the outsides to be the gun platforms and finally added guns. I also plan on building at least one for 40k, should be a lot easier and cheaper to build than a titan. The wheel will be rather easy, I'll just need to custom build the platform, and then buy 2 Leman russ battle cannon turrets, and 4 defiler reapers.
Chaos warhound(or Feral) titan: The forgeworld one. 3 structure points, 2 void shields, pick your own weapons.
Chaos reaver(or Ravager) titan: Same as a reaver but tends to have a tail added that can be used for crushing in hth but also has a gun built in for shooting. Weapons I'm not sure on, but the tail's gun is the equivalent of a battlecannon. 6 structure points, 4 void shields.
Chaos warlord(or Banelord, Plaguelord, ect.) titan: Same as a warlord but like the ravager it has a tail with a battlecannon equivalent built in. All sorts of nasty waepons. 8 structure points, 6 void shields.
Most of these were gathered from: Epic black legion rules