To get a Venusaur on Pokemon Diamond you have to get the National Pokedex and look for its area, if it's unknown then I don't know what to do.
You must migrate Venasaur from FireRed/LeafGreen to Diamond.
migrate it from pokemon 3rd generation or trade it from some one
A female Venusaur and any grass or poison type male.
Venusaur is a Pokemon. It is the last evolution of the Pokemon Bulbasaur.
Venusaur is a Grass and Poison type pokemon.
venusaur!?
Venusaur is a Grass/Poison-type Pokemon.
Petal Dance is not a TM in Diamond or Pearl. If you want to have one of your Pokemon learn this move, the Pokemon would have to be a Venusaur, Oddish, Gloom, Vileplume, Meganium, Sunflora, Roselia, or a Cherrim.
Infernape Venusaur Blastoise Palkia and 2 others
breed it with a ditto or a venusaur of the opposite gender
Yes your rival will if you choose squirtle, i have a venusaur.
A Venustoise? half Blastoise half Venusaur? I don't think it exists, go on Youtube.com look up "Venustoise" i think there's two Videos about it, my best guess is to put a Venusaur and Blastoise in the Pokemon daycare and see what happens, but make sure the two Pokemon are the opposite gender.