The only reason megahorn is bug type is because it was Heracross' signature move. It could just as well be a normal type move like horn attack.
Similarly, horn leech is Sawsbuck signature move, so it makes sense why it's grass type as that makes it a STAB move for Sawsbuck.
As for leech life, 2/3 of the families that could learn the move in gen 1 are bug type. And I guess they needed more bug type moves anyway because the only other bug moves were twinneedle and pin missile.
Similarly, horn leech is Sawsbuck signature move, so it makes sense why it's grass type as that makes it a STAB move for Sawsbuck.
As for leech life, 2/3 of the families that could learn the move in gen 1 are bug type. And I guess they needed more bug type moves anyway because the only other bug moves were twinneedle and pin missile.