It depends if they are CITES listed animals or not. What species are you thinking of?
I called the border a head of time and asked what they would want to see when I came back into Canada. I brought several books with pictures of the snakes to ID them and then they looked them up in some HUGE book to see if they were ok. It was that easy... at least that time it was. I was just bring some kingsnakes over though and I haven't tried it again since.