Building a Rolling Menace

[mint]

I have an opponent for my PCs in mind and I am not sure how to build it.
The idea is a small creature that turns into a large ball (inflates kinda).
I want it to be mobile and do stuff with trip and deal some damage by running over the PCs.

Fluff:
The PCs are fighting off a siege from a goblin army roused by a religious figure called the rolling saint. In the army there is an order of zealots devoted to the rolling saint. Clerics who are good with tumbling and use spiked chains. This NPC is the leader of these holy dudes. It seemed appropriate that she would be able to embody the principles she devotes herself to, rolling. So she'll inflate herself to large size, and act like a big angry bouncy ball

What I have in mind so far:
ECL 10
Synthesist Summoner (PF), Quadruped with large, trample or pounce and trip evolutions.
I don't mind if isn't completely legal :3
I try to make legal builds when I build things I am not sure about to make it easier to gauge power.

Any ideas?

[CTrees]

Use the spell Summon Eidolon to hit your surprise small goblin -> large creature. Enlarge Person will then get you to huge, though you could perhaps create a "Greater Enlarge Person" known by your goblin synthesist (likely a third level spell for him) to give a two size category boost, going to gargantuan. If you really need to, that is - an elephant-sized ball of angry goblin (you could fluff away the legs and make it just a big ball, if that's what you really want - just have them give the mechanical boost only) is probably sufficient.

[Malimar]

You might take a look at the Ascomoid from Dungeonscape for ideas. It's basically a ball that attacks by rolling over you.

Probably the most relevant things to take away from it: Trample; the ability that lets it do Trample damage despite not having a slam attack; the ability to automatically trip anything it tramples; the immunity to trips, immunity to being prone, and -4 to resist being bull rushed.