Horse is unique - it starts Large, so classes built around Mounted Combat could ride it from lvl 1, and does not grow one step bigger at lvl 4, when it gets stat increases like all pets.Also why is that horse is the only one that doesn't grow in size? I didn't play WotR long enough to know.
I think biggest mechanical point of this guide - looking at the size, like Small/Medium/Large, or even at pet model could be misleading, it is all about the size of the green selection circle. So even if numbers themselves could turn out not to be applicable to Kingmaker, it has solid idea about what you should look at to compare them properly.