This question is more around the fun of playing decks casually. Horsemanship I believe is legal in Commander and Legacy. However, if you wanted to enjoy some casual games with a Portal Three Kingdom deck against some Modern or Standard decks, would it be necessary to treat Horsemanship the same as Flying and allow cards that have Reach or cards that counter the Flying ability to be used against Horsemanship?
What you suggest was certainly thought about by WotC when they allowed the Portal cards into Vintage and Legacy:
Should we errata it so that horsemanship creatures can be blocked by fliers? In the end we realized that we were kidding ourselves—the creatures with that ability just aren't aggressive enough to worry about, so we left the ability alone. If you want to play horsemanship in constructed, knock yourself out.
so it's not an entirely stupid suggestion - but in casual games, your best bet is probably just to try it and see if they feel overpowered; the answer will probably depend enormously on the power level of the Standard/Modern decks you're playing.