... Or, for that matter, any other creature it doesn't like?
Pathfinder's rules for mounted combat seem to ignore creatures of Tiny size (which typically have a reach of 0 ft.) riding creatures of Small size (which typically have a reach of 5 ft.). Like this:
Must a Tiny creature wielding a typical melee weapon dismount and enter a foe's square (usually provoking an attack of opportunity from the foe) to make an attack, or can such a Tiny creature while mounted on a Small creature attack a foe without entering the foe's square? What about the same Tiny creature wielding a reach weapon? (This latter gets ugly because of the way reach weapons work (i.e. they, apparently, don't) for Tiny creatures, this despite, for example, a lance-wielding monkey seeming perfectly reasonable... in this context anyway.)
If it's determined that the rules are as they seem (e.g. a monkey wielding a lance riding a dog must dismount, enter a foe's square, then attack a foe), I'd appreciate either a house rule to fix this lamentable situation or an explanation why no fix should be implemented.
"Why would this ever be a thing?"
Einrich Trogen, a human Fighter 2 (eldritch guardian archetype) / Hunter 8 / Mammoth Rider 3, usually fights mounted on his mastodon companion, Fleetwood, and Trogen possesses several feats that make him an adequate mounted combatant. Trogen's monkey familiar, Nielsen, usually shares Trogen's feats, but lacks a special mount, instead riding a Buddy, a common dog with combat training. Nielsen, because he shares his master's feats, is an excellent rider and mounted combatant, but his 0-ft. reach confuses everyone.