Basically, according to his theory, triceratops is a juvenile form of another species, the torosaurus. Now don't get into one of those "Pluto was a planet when I was a kid" panics -- because the species was named triceratops first, that's the name that will stick.
In case you're completely lost, here's a triceratops on display at the American Museum of Natural History in New York:
And here's a torosaurus at the Peabody:
So, as I said, Horner's idea is that the Torosaurus is the grown-up version of the Triceratops. So think of Triceratops more like this:
Upon hearing Dr Horner's theory, my mind immediately turned to Denver's most famous triceratops, Colorado Rockies' mascot, Dinger:
Dinger's origins as a living triceratops in the late 20th century are somewhat murky, but news stories from around the time mention that, during the construction of Coors Field (where the Rockies play), a number of dinosaur skeletons were found and subsequently excavated. My best guess, therefore, is that Dinger is the result of that excavation -- a clone of some dino DNA from a skeleton found there, mixed not with West African frog DNA, but with the blood of the Blake Street Bombers...
So here's Dinger, and in the words of Alan Grant, he's out of his time and confused. And yet he still goes out there and does a great job every day, even though some fans seem to really dislike him.
Now what does all this have to do with Dr Horner's theory? Well here's my concern: why hasn't Dinger, who is almost 20 years old now, grown into a torosaurus and lost his third horn? Why hasn't his frill elongated and developed those characteristic holes? And here's MY theory: remember how I said people don't like Dinger? Well one of the charges leveled against him is that he's fat, and therefore a bad example to kids. My concern is that the reason why he hasn't aged into a torosaurus -- and the reason why he's kind of chubby -- is that the Rockies organization is concerned about him reaching sexual maturity and going on a violent dinosaur mating rampage (since many palaeontologists have theorized that those head frills were for clashing over mates) and therefore is feeding Dinger hormones in order to stop his growth. A side effect of the hormones is weight gain.
Just a theory of mine.
Non-sequitur: Wikipedia claims that "Dinger" is the ancient Sumerian word for God. Citation needed.