- Speed doubles in rain.
- Raises Speed one stage when hit by a Bug-, Dark-, or Ghost-type move.
Level 100 Statistics (see level 5, 50, 100)
There is no excuse for using Magikarp, at all. All comedy value in using it disappeared by about 1998, and anyone who finds it funny deserves to be fed Rare Candy until they get diarrhea.
With the nerf of Explosion and Selfdestruct in Gen 5, Magikarp lost out in its main use as a sacrifice to switch in when the opponent goes boom. However, it still hits a respectable 426 with Jolly and Choice Scarf attached, so you might be able to revenge kill a low-health Dragonite or something. Putting Sandstorm up would do exactly the same thing for all the damage you're dishing out, but let's work with what we have here.
Focus Sash is good for making Flail work, especially to lead with. The other items are only mentioned because they are all just about as useful overall.
Team Options & Additional Comments >>>
You could maximize HP at the expense of Speed or Attack to turn OHKOs into a 99% chance for OHKOs. You also might as well maximize Special Attack, if only to troll people rating your team. Alternatively, you could just evolve it into Gyarados. Or use any of the vast number of Water-types available. Seriously, there's like a hundred of them and you picked this one?
Checks and Counters
Stop using an Old Rod.