Hopefully this will be the official one so I can stick all of my findings into this thread.
This one is an anthro snake TF that just happened spontaneously on the woman or cartoony style.
http://www.furaffinity.net/view/9307972/
The next one I found is a serious of sketches involved possession TFs that where the host enters the body through many means... normally a vagina.
http://www.furaffinity.net/view/9348723/
Although, after I've seen this TF
http://www.furaffinity.net/view/5658281/ from Fox0808. I'm curious about seeing a possession TF where the process happens as the host is going into the victim's body.