Neph, I must disagree about the fatness. (Although at this point the argument is kinda moot as it's the DM's call, I'm just sharing my opinion, pls don't take it as an offense, I just like beating dead horses)
Speaking of horses.. Lets take a look at them.
clydesdale - 6 feet tall, 8-9 feet long, 2000 pounds
Marilith - 9 feet tall, 20 feet long, 4000 pounds.
Clydesdales are not FAT, they're loaded with muscles. The marilith is Bigger than a Clyde, and arguably even more butch (Being able to bench press over two tonnes!).
True you could point out that she's not built in the same shape as a horse (she's more long than tall wile the horse is square), but then I point out that most of the horses size is taken up by its legs, which account for very little of its mass. Lets look at just the torso of the horse. It's about 6' long, 3'X2', and weighs say 1800(I can't see its legs/head weighing much more than 200 pounds).. That's 36 cubic feet of muscle, weiging in at 50 pounds per cubic foot.
The Marilith, even if you just assume her as 20' long, 3X2 wide, ignoring arms/head (Which I think is an underestimation), she would have 120 cubic feet. Now, assuming she's built HALF as solidly as that horse, that gives us 25 pounds per cubic foot, or 3000 pounds. And that's on the Low end of what I think is an underestimation of her size.
In essence what I'm getting at is that I think you're viewing her as a human with a snake tail, wheras I'm looking at her as a 9' tall butch demon whos lower half is a 20' long scaley coil of muscles.
She can Use Magic Item to cast Summon Monster IX from a scroll. Colossal Fiendish Spider ftw . It can already carry much more than her 4000 lbs, so that's not an issue. It will obey her commands and she has the stat bonuses to be able to ride it.
You also realize that it would only last for 17 rounds, right? It's something that you'd have to spend the first standard action of every combat doing, in order to gain a move action...
And then there's the question of whether or not snake lady can even 'sit/mount' the thing.
But I want to be able to use something less over the top.
Probably a good idea, I just had an amusing image of a
Marilith riding into combat on a
Fiendish spider shouting "I'M NOT EVIL I SWEAR!!"
I still say if you're looking for mobility, Belt of battle + Quicken SLA: Teleport make more sense and are just as(If not more) effective.