Mentor says I cannot be first author of my paper because I am an undergraduate I performed some research and submitted it to a congress of biomedical sciences. Many professors told me that my research is very advanced compared to student work and that it could easily be my specialization one day.

I am an undergraduate student (sixth year veterinary science).
My mentor tells me that no journal will accept me as a first author although the entire research is mine. I did everything except the statistics, because by the rules, the mentor must do it. I spent four months on the field, alone, taking samples, and an idea of gifting my own blood, sweat and tears is very painful.

Do you think I should wait for a year or a bit more to publish it as a graduate or should I let them take credits for my work. The research is about horse welfare in my country and it has never been done before. This means a world to me. I want to devote my life to this area and publishing my paper as mine would be only right thing to do, right?
Should I accept publishing the paper with someone else’s name as a first author? 

Update

My mentor yesterday late night sent me a message informing me that they (she and her head of the department) discussed my work and agreed that probably I will be named as a first author, since I did all the work. 
Thank you all for answers, I wouldn't be so confident on this matter without your advices.
My mentor tells me that no magazines will accept me as a first author


This is clearly wrong. Of course I cannot speak for every journal in your particular subfield, but rejecting papers only because of the academic rank of the first author would be ethically dubious. At the very least, there are some megajournals that do not have such criteria and which should accept your work – given that it is scientifically sound, etc. (Mind that I am not saying that megajournals are your best or only choice; they are just the easiest counter example.)

Moreover, a typical journal won’t even ask for your academic rank and may not bother to investigate it and neither do the peer reviewers. The journal may even practice double-blind peer review, in which the reviewers won’t even know your name and thus probably lack all means of finding out your academic rank.

Finally, while there are some people who judge a work by whether they know the authors (though they shouldn’t), they usually do not look at authorship positions. If somebody is going to be more benign to your work because your mentor’s name is on it, they will likely not care whether they are the first or last author.


  by the rules, mentor must to [the statistics]


There are certainly no universal rules for this. Where did this come from?



Anyway, your problems rather are these:


Doing the statistics almost certainly qualifies your mentor for authorship, so he must agree on all publishing decisions. (The opposite also applies.) Even if you re-do the statistics, it will be difficult to argue that you did so completely independently. Also mind that any intellectual contribution qualifies for authorship, so think about any feedback or advice you got from your mentor on this work.
Your mentor may have some power over you. I don’t know them, but you must consider the possibility that they freak out over this and try to sabotage your career and particular this very paper.
The biggest advantage of having an experienced mentor contributing to your paper is their experience in writing. It is very unlikely that anybody produces proper scientific writing on their first attempt without help from somebody experienced with it.


Unfortunately, you have to judge your mentor’s mentality yourself.
However, unless you are sure that they do not abuse their power, I strongly suggest that you find another professor or similar who can advise you and possibly act as an arbiter in this situation.