The EN World kitten
In short I don't like the Tren, they could be replaced (fairly easily) with some other creature that serves the same purpose, and that already exists in a monster manual or supplement somewhere. Please keep in mind I have deliberately not Googled the Tren, so I'm bound to be wrong about this.
So as it turns out...
The tren are from MC11 Monstrous Compendium Forgotten Realms Appendix. Insofar as I'm aware, they've never been used in any other capacity besides this novel (which I haven't read, and so didn't know about). For what it's worth, I appreciate Cunningham trying to pull a lesser-known monster out of obscurity, though I suspect that I'd have gone crazy at the time trying to figure out just what those things were and what they could do.