Monsters like the Siege Crab

[Nodsiu]

In the MM3, there is a Monster called Siege Crab. One ability it has is a compartment other people can go inside.

I'm trying to find other monsters that has ability like this, so if you know of any please tell me.

[Inevitability]

The Elsewhale, from the Planar Handbook, can carry up to 4 medium (or 1 large, or 16 small) creatures in its mouth.

Also, the Arms and Equipment Guide has rules allowing one to ride in a special cyst in the mouth of a purple worm.

[ShurikVch]

Body Pouch feat (Serpent Kingdoms) allow to
part your scales to reach a hidden, scale-lined, flexible cavity in your body that can accommodate objects or creatures. The pouch can hold a maximum volume equal to that of a creature two or more size categories smaller than yourself, and its maximum weight capacity is one-half your own weight.
Any creature inside your body pouch remains alive and comfortable for up to 24 hours unless you choose to attack it.Since "Scaled One" prerequisite includes the DINOSAURS, I presuming Body Pouch of Colossal reptile should be able to hold the whole party

[Zanos]

Anything with swallow whole could technically do this, if you can become immune or resistant to it's swallow whole damage. I think some monsters don't do any.

[unseenmage]

Intelligent Magic Item Apparatus of the Crab could work. Works even better if the levers count as a 'special power' that the Intelligent Magic Item can use on its own.

Animated Object or Minor Servitor (SS) on a building or vehicle would also fit.

[Nodsiu]

Thanks everyone is helpful.

[ShurikVch]

Also, Amulet of Ooze Riding (Arms and Equipment Guide)

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS5iAFa94Qfaoexq1CVGF7aS_-6IFZ14hTPzREmaCs9g4zA2DugfA

[Allanimal]

Maybe not quite what you're asking, but Dalmosh has an entire plane as its stomach...

[Inevitability]

Maybe not quite what you're asking, but Dalmosh has an entire plane as its stomach...

More like a planar (one-way) portal in its actual stomach. Once you're through, your location is no longer dependent on Dalmosh's: you can't easily return to its stomach and if you Plane Shift away chances are you won't reappear near Dalmosh.

[chainer1216]

Just stopping in to say i love the siege crab, its so weird and cool.

Our partys dread necro raised one as a skeleton, we put in hardwood furnishing in the compartment, attached the crab to a massive armored carriage and use it to travel around.