Hriston
Dungeon Master of Middle-earth
Intelligence is a funny beast. I've had people (not just one) seriously argue that the INT 6 and understanding a language steed from the paladin Find Steed was still non-intelligent, because there was no definition of "intelligent" in 5e and a INT 3 human (minimum roll) was intelligent but an INT 6 beast was not. This came up because of the section in the PHB about intelligent vs. controlled mounts.
I'd say 4 was the minimum INT to be considered an intelligent creature for mounted combat purposes. So you would only have the option to control a non-intelligent (INT 1) mount or a mount with animal intelligence (INT 2-3), provided it has had the required training of course. As for the paladin's steed, the spell itself describes it as intelligent.
I bring this up not to say this endeavor is doomed (DOOMED!), but to think about if your ratings still make sense when applied around, both withing the normal ranges for humanoids and also when applied to non-humanoids. For instance, if generic people follow the 3d6 bell curve, then about 1 in 8 is INT 7 or less and about 1 in 20 is INT 5 or less. I don't think we'd say 1 in 20 people are best described as "semi-intelligent".
As I mentioned in my OP, my ratings don't assume a general population that follows a 3d6 distribution of scores. The 'averaging' method of ability score generation is assumed for the general human population, i.e. 3d{2, 3, 3, 4, 4, 5}. Under that distribution, less than 0.5%, or 1 in 200, have an INT of 7 or less after the human ability score increase is applied.