When writing a text on my iP4, if using my spell checker, since the latest software update, it is automatically dropping a capitol "Z" at the start of each word that is spell-checked. It is getting really enoying now as you have to go back an delete the letter every time.
No such problem here. If you touch the small "x" on the correction bubble, the dictionary should learn your spelling. Also try:
Settings > General > Reset > Reset Keyboard Dictionary.
By any chance have you been downloading a lot of zombie games, apps, videos, etc.? iPhones don't like zombies or any reference to them - the phone may think you've been bitten and converted.