How can I sketch in the Messages app
This is not your just your memory. In response to your question that feature requires the jail broken platform "Cydia". The specific feature is called Graffiti and you need 2 or more languages enabled on your device's keyboard for it to work, once you have that hold the globe icon on you're keyboard for it to work.
For more info I found this site and I think it explains best here's the link Hope it helps :)

http://www.redmondpie.com/how-to-compose-texts-or-imessages-on-iphone-with-drawings-handwriting-and-sketches/