In Stadium, the user becomes uncontrollable during its next turn. Otherwise, the user attacks on turn one but it doesn't recharge on turn two if it KOs the target, breaks its Substitute or misses entirely.
The target is badly poisoned, with the damage caused by poison doubling after each turn. However, Toxic poisoning will revert to normal poisoning if the poisoned Pokemon is switched out or uses Haze. Damage caused by Leech Seed will also increase as Toxic damage increases.
This move is temporarily replaced by one of the target's moves; after one PP is subtracted to use this move, the replacement move will retain Mimic's remaining PP. The user can select any of the target's moves to copy until the end of the battle or until the user switches out. There are no move restrictions on Mimic other than itself and Struggle. However, in a link battle, Mimic will copy one of the target's attacks randomly.
Normal
N/A
100%
Other Options: Slash (at the expense of Swords Dance), which will always CH, and Counter.
Verdict: Scyther is probably the "best" Bug in RBY, but keep in mind that Bug was the worst type in RBY. Swords Dancing and mauling things with Hyper Beam is the extent of Scyther's usefulness and it might as well not even have third and fourth moves because its other options are that bad. Toxic is pretty much the definition of filler since Swords Dance and Slash make a horrible combo, but at least you might Mimic something decent or Quick Attack might kill something weak for you... Scyther's limited movepool and bad typing just wreck him in this generation.