I understand the title may be confusing, but let me explain. JK Rowling recently (I believe) said that she thought it was funny that almost no one noticed the snake in the first movie was Nagini, the snake Harry kills in the last movies.
If you were just watching the movies, there is no way for anyone to have noticed this. The people who rendered the snakes did completely different species!
The snake from the first movie clearly resembles a burmese python, while the other snake is unmistakably a retic.
First movie
http://thechive.com/2011/11/16/daily...ump-dar-36-14/

Seventh movie
http://harrypotter.wikia.com/wiki/Nagini

While I do have great respect for the artist actually making the CGI for real snakes, it is equally ridiculous for them not to even do the same species.
I also think that in the seventh movie they switched between a regular retic and a tiger, but I'd have to watch it again to be positive.

Thoughts?