Goristro are the giant 20'+ living siege engines of the blood war, big stupid brutes whose key ability is "take piles of damage" and "dish it out". Besides that fairly simple and basic concept, there's a few excellent little tidbits about them.
One of these is the existence of "goristro citadels", which are platforms strapped to the head and shoulders of the goristro, much like a howdah, from which higher-ranking tanar'ri can surveil and command battles. That's awesome, and I know I overuse the word, but still. If I ever have a planescape character from whom it becomes relevant, they're going to want one of those. I mean, there's logistical, and ethical, concerns with having a two-story-tall carnivorous demon steed, but you still have to admit that's a style that just can't be beat.
The other interesting detail is that goristro breeding isn't done naturally, but rather "they are carefully mated by their owners after extensive negotiations". Goristro breeding is evidently a big deal, with a lot of underhanded skulduggery and betrayal going own with these arrangements, involving slaying the other side's goristro or stealing the young or such things. So it's like dog breeding, only with giant two-story-tall carnivorous demons, and it's also being run by other demons. Sweet.
* If the PCs are the sort who work with tanar'ri, reward their open-minded tolerance with an offer of employment overseeing and securing a "negotiations of a delicate matter and the resulting proceedings". It will, of course, mean they'll be watching over two goristro mating. And yes, there will be foul play. Okay, this one's kind of immature and comedic, but with plenty of potential for old school violent action regardless.
* The PCs raid a slain Baatezu's treasure vault, and find several old war trophies: including a few goristro collars and wrist bands. Of course, once the wards of the vault are broken, the bands' functions as magical trackers become active again, alerting their former masters as to their location right as the PCs carry their new ostentatiously bejeweled loot away.
* A retired adventurer recalls an old fancy: owning a personal goristro citadel. He's quite wealthy now, of course, enough to hire some young up and coming heroes to do the dirty and bothersome work of capturing a goristro alive ("The bigger, the better!") and transporting it to an already purchased plot of land in the Outlands where he'll build his 'stable'. Piece of cake, yeah?