It's actually Dawns sister. If you want one, PM me.
If you chose to be a girl at the start, it's not Dawn's sister - it's Lucas' sister :p
Tropius, Tangela, poochyena and the previous gen starters and legendaries are unavailable and must be pal-parked in.
No. Tangela is a swarming Pokemon, like I said.
Tropius is in the great marsh safari game. You need to view it through the binoculars before entering the safari game after you get the national dex.
Mightyena is a Pokeradar Pokemon exclusive to Diamond on Route 214 and 215. Catch and breed it to get Poochyena. Pearl players require a trade or Pal Park from Hoenn.