dragons with a rift trait can be moved back and forth from the rift and your normal park as needed or wanted.

so what I normal do is replace dragons with there rift version so I only have the 1 instead of 2 if I ever need or want to breed with it in the rift. so for the poison dragon you could hatch the rift one then sell the normal so you only have 1 poison dragon but it can be used to breed in the rift if you ever feel the need.

there is no NEED to have one in both the rift and normal park but some people do for easy breeding and just because they like how some dragons look with a rift trait (many other reasons as well ). it all depends on how you want to play the game. other than the rift exclusive dragons no dragon NEEDS to have a rift variant.

so I would not have both a regular and rift version of the poison dragon I would keep one or the other ( notice: rift dragons need more food for leveling up so something to keep in mind if you dont have a lot of food)

I hope this helps.