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It can learn it, you just need a Heart Scale because it gets it from the Move Relearner. He's located in Blackthorn City, in a house near the Poke Mart.
There's no way to legitimately obtain a Pokemon at Level 1 unless it's through breeding, and Heracross is mentioned to learn Night Slash at that level. Some Pokemon that learn moves at Level 1 only learn them from Move Relearners so that's why I mentioned him, because Heracross will likely only learn the move in that method.
Heracross learns 5 moves at level one. Night Slash is listed in-game as the first move, and because wild Pokemon movesets are based on the most recent four moves, no it is not possible.