Someone claimed me as a dependent, idk who it was, I found out because my tax return was rejected, after my tax got rejected, I created an IP pin, would their tax return get rejected because I have an IP pin now or is it too late?

If someone used your SSN on a tax return, the reason your own return was rejected is that the other return was accepted.    The IRS does not "reject" a return after acceptance, so, no, that other return will not be rejected now.   But since the IRS will see two returns in the system using the same SSN, the IRS will sort out the duplicate use of the SSN's.   Hopefully your return will be accepted with the IP PIN; but do not be surprised by a letter from the IRS someday when they are sorting out why your SSN was used twice.  

If your e-file using the IP PIN is also rejected, you will have to file by mail.

Now that Ip pin, will theirs still go through? or will their return get cancelled because the IP pin that I created.

Their return cannot be canceled once it is accepted, as xmasbaby0 points out.  

You and the other taxpayer will get letters in the latter part of the year with near certainty asking about the duplicate filing.  If it is because someone claimed you as a dependent, you'll be asked to show evidence that you were not.