Is it considered rape or some kind of sexual assault if anal sex was forced against your wishes? If you and your partner were already having Intercourse (vaginally), but you did not want to do anal, that's your right to say no & not want to do it...right?

Omg  yes that would be considered rape! He has NO RIGHT to do that. The only problem would be you might have difficulty taking this to court as it is sometimes difficult to prove what happened wasn't consensual, even if there is signs of violence he might say you asked for rough sex.

I know because something similar happened to a friend of mine where they were in bed together but she didn't want to do anything just lay in bed but he forced her into some things but it was impossible to prove for sure.

But you should DEFINITELY still report this. Because he needs to know this is not ok, and the police need to know he's this type of guy. If he gets away with doing it on you, you can be sure he will force it on other girls too.