No. They were made after Pokemon Yellow.
Trade them from HeartGold and SoulSilver.
yes you can also catch them in soul silver.
suicune is top legendary dog just play Pokemon crystal and you'll see.
EnteiSuicuneRaikou
If by enti you mean Entei the legendary dog then yes, all three dogs are there.
Groudon, Kyogre, and Rayquaza. Groudon is the fire legendary pokemon, Kyogre is the water legendary pokemon, and Rayquaza is the air legendary pokemon.
entai- legendary dog Pokemon
lillipup
Yes you can.
try to catch your legendary. It varies on which legendary depending on what starter you chose.
Nothing happens after that. You just caught the three legendary roaming Pokemon in the Johto Region.