I was just reading about Bilbo and the three trolls: William, Bert, and Tom. 

At the end of the quote, there is a reference to multi-headed trolls.  

> William choked. "Shut yer mouth!" he said as soon as he could. "Yer
> can't expect folks to stop here for ever just to be et by you and
> Bert. You've et a village and a half between yer, since we come down
> from the mountains. How much more d'yer want? And time's been up our
> way, when yer'd have said 'thank yer Bill' for a nice bit of o'fat
> valley mutton like what this is." He took a big bite off a sheep's leg
> he was toasting, and wiped his lips on his sleeve. 
> 
> Yes, I'm afraid trolls do behave like that, even those **with only one
> head each.**  
>     - *The Hobbit* p.35

Does Tolkien further describe multi-headed trolls in his notes, or is the reference just a *one off* limited to *The Hobbit*?