Not a pokemon. If you're talking about regigigas the you have to have all 3 regis (regice, regirock, registeel) and you have to take them to Snowpoint temple.
there is no Pokemon called regigrass at all
You should get him Dialga Palkia Regigrass Darkai Heatran Cresselia and if you want more you can visit this site and you will see DP and look under that http://www.serebii.net/pokedex-dp/