Context: a man reads some online posts after accusing another man of being a pedofile.
What is the meaning of 小動物系? Considering the context and the fact that images of young girls came out when I tried to look for it on Google Images, I think the meaning is not simply "small animals system".
Also, could you explain the meaning of 裏ＤＶＤ and "こマ!？"? I think that the first means "illegal DVDs", while te second sounds to me like "really?!". Is it correct? Thank you for your help!
小動物系
In this case 系 doesn't mean system. Instead it refers to a certain category of girls. 小動物系 describes girls you could compare to small animals in terms of cuteness. They obviously can't be tall or overweight, but they also must behave in a certain lovely manner. You can read more about this use of 系 here.
裏ＤＶＤ
You're right, these are DVDs that can't be sold in the open. Specifically, the term nearly always refers to sexual videos.
こマ!?
This is a shortening of これマジ!?, which indeed means really?! \ are you serious?!.
