I just want to point out real quick that I was writing my post as you were yours so I edited mine to include an answer to yours. (Just FYI since it's an edit on another page)
The idea behind Sub is that, like RP, it allows you to get past Heracross. (who is OHKO'd by SE) The advantage of Sub though is that it punishes you less for a mispredict. RP has to be set up on a predicted switch-out so a mispredict costs you Terrak. However, Sub does the same thing (Terrak outruns everything except Heracross who dies popping the Sub) except that if you mispredict and the OP decides to stay in and attack you only lose 25%. The only downside I can see so far is when Terrak tries to set up on a locked Megahorn. With RP he either profits from the switch out and prevent any future attempt to revenge or it outspeeds after tanking a Megahorn and OHKO's right after. Sub on the other hand has to be used on a switch out to be effective as if it is used when Terrak stays in then you lose an extra 25% KOing Heracross as it pops your Sub. Even a healthy Terrakion only has enough HP to make 3-4 more attacks after that mispredict. Although that mostly means 3-4 kills so that isn't so bad and a choice locked Megahorn is less common than a predicted switch out. I'm kinda up in the air on this but I'm leaning towards Sub.
252 Atk Heracross Megahorn vs. 0 HP / 4 Def Terrakion: 104-123 (32.19 - 38.08%) -- 96.63% chance to 3HKO
The idea behind Sub is that, like RP, it allows you to get past Heracross. (who is OHKO'd by SE) The advantage of Sub though is that it punishes you less for a mispredict. RP has to be set up on a predicted switch-out so a mispredict costs you Terrak. However, Sub does the same thing (Terrak outruns everything except Heracross who dies popping the Sub) except that if you mispredict and the OP decides to stay in and attack you only lose 25%. The only downside I can see so far is when Terrak tries to set up on a locked Megahorn. With RP he either profits from the switch out and prevent any future attempt to revenge or it outspeeds after tanking a Megahorn and OHKO's right after. Sub on the other hand has to be used on a switch out to be effective as if it is used when Terrak stays in then you lose an extra 25% KOing Heracross as it pops your Sub. Even a healthy Terrakion only has enough HP to make 3-4 more attacks after that mispredict. Although that mostly means 3-4 kills so that isn't so bad and a choice locked Megahorn is less common than a predicted switch out. I'm kinda up in the air on this but I'm leaning towards Sub.
252 Atk Heracross Megahorn vs. 0 HP / 4 Def Terrakion: 104-123 (32.19 - 38.08%) -- 96.63% chance to 3HKO