Huge companions in dungeons

[Vodoff]

Situation is this: have a huge undead mammoth, suspect that we are going to be going through dungeons at some point, and however you look at it you cannot get a 15x15 creature through a 5x5 tunnel or door normally. Also don't want to leave it outside because that makes it susceptible to DM doing whatever they want to it, like "A roc came and dropped it repeatedly, then took its unmoving corpse away."
How do I a) get a mammoth through a tunnel and into a dungeon (preferable since character is primarily utility focused, mammoth is the only combat ability) or b) store it outside a tunnel in a way that it is not going to be cheesed.

[VoodistMonk]

If you have to worry about your GM messing with your stuff, it is probably time to find a new table...

[Agénor]

This is a meta-game discussion to be had with your Game Master, about how relevant a defining feature of your character is going to be in the campaign. Making a character having invested significant resources to combat the undead when there will be five encounters with them over the course of twelve levels isn't fun, make an artic druid when most of the game is in a jungle, making a character with an animal companion of size bigger than ordinary....
Talk to your Game Master, this is a co-operation between the player and the G.M., not something to be sorted on the level of the character.

[Java Man]

Burying undead is an okay temporary storage method.

[Quixote]

That's part of the natural drawback of especially large companions/minions. They're stronger and tougher and more potent in combat, but they have mobility issues.

I'll agree with VoodistMonk in part; if you're worried about your GM taking advantage of every little thing, there's a bigger issue here.
But then again, that is *sort of* the GM's job. They're supposed to mess with you. With your hit points and resources and all that. If you invest a huge portion of your resources into one thing and that one thing gets negated in some fashion, that isn't automatically because you're GM is being a jerk. My players never build one trick ponies because some of their foes are smart enough to make over-specialization a liability.

As Agénor said, this is definitely something to talk with your GM about at session Zero. Focusing on types of enemies, terrain, equipment or even gameplay styles should only be done when you have word from the GM that it won't be a terrible idea.

[Firebug]

Rope Trick(2nd) will protect it from random non-intelligent monsters. Shadowy Haven(4th) during the day(or night) will likely protect it against even intelligent monsters.
Getting Mammoth to climb might be difficult though. The wording on the spells implies that you don't have to worry about the creature fitting through the entry, because they explicitly allow the storage of a bunch of creatures of any size. But your mileage may vary.

[Sysryke]

Invest in some scrolls or wands of "Reduce Animal" or whatever is appropriate since it's undead. This is an AC correct? Familiar feats won't help, but depending on how house rule friendly your game is, you might be able to come up with some equivalencies to familiar pocket, or tatoo familiar or some such. Alternatively, try for various incorporeal or gaseous form type spells. Over-sized critters require finaggling.

[TxSam88]

buy Hosteling armor.

https://www.d20pfsrd.com/magic-items/magic-armor/magic-armor-and-shield-spe cial-abilities/hosteling/

[Java Man]

Hosteling armor only handles up to large animals, and doesn't work on undead.

[Mark Hoover 330]

So, this is a Huge sized undead mammoth, and the title suggests this is an Animal Companion? Maybe see if your GM will let you get away with the Carry Companion spell. If not, find an extra dimensional space with a really big opening. Barring that, or any of the other remedies upthread, you may have to custom make a magic item for the mammoth to wear that alters it, perhaps with the Shrink Item spell or something. One other option, see if the combo of Share Spells and one of the Beast Form spells might work.

[avr]

If the dungeon is a building there's the possibility of making a door.

Less structurally dangerous, the gaseous form spell works on willing corporeal creatures.