You can defeat the Elite Four again and again (you can get stars on your trainer card for this), and you can fill up your pokedex. You can also try to catch a shiny pokemon (you have a 1 in roughly 8,200 chance of seeing one in the wild). You can explore pokemon breeding to get different pokemon to learn many unique moves.
It's highly recommended to use it on your legendary dog Pokemon whether it's Raikou, Suicune or Entei.
You have to trade for it.
moltres
no but you can in emerald after catching 202 Pokemon
yes but it depends on who and what you are fighting
The legendary dog you find the dog that has a type advantage over your starter.
just go to the pet shop and buy all of them.
they roam around in the wild after you have done everything on the islands but they run away
You would have had to chosen Charizard as your starter Pokemon. Whichever legendary dog appears depends on your starter. You will find your legendary dog randomly in the wild. (you must beat the elite four first)
Entei, Raikou and Suicine are often referred to as the Legendary Beasts or Legendary Dog pokemon. They aren't in Diamond, so you'll have to trade or migrate them. You can do this from the following games: FireRed/LeafGreen, HeartGold/SoulSilver and OmegaRuby/AlphaSapphire.
you have the best chance of finding and catching raikou(or any of the legendary dogs) on route 1.(use your master ball. raikou can run away.)
It's highly recommended to use it on your legendary dog Pokemon whether it's Raikou, Suicune or Entei.
No
No
you can beat the elite four for the second time and complete the national Dex
You have to trade for it.
It is level 50, as are all 3 legendary dog-lions in Fire Red and Leaf Green (Suicune, Entei and Raikou).