Is it also "rata" masculine? Is is correct to say "un rata"?
I have read the expression: "Eres un rata". It seems to me that is a wrong use of the undefinive article "un". For me should "una" considering that "rata" is femenine noun.
4 Answers
rata is a femenine noun, it's "La rata, Una rata Las ratas". In spanish we use the word rata for referring a person who is a coward (because the rats are the first to leave the boat) or a thief, for expample "Eres un rata, El es un rata" to refer to a man. Or maybe the person who wrote was wrong.
I hope that you understand because my english isn't good 
Suerte!
OK, There are many synonyms for thief, one of them is "ratero", so instead of saying: eres un ratero one says eres un rata, which is very similar but more peyorative and sometimes just to make fun.
Raton is masculine nad rata is femenine,eres una rata .
No,i do not believe it is wrong, i think it CAN be writen un in some ocasions as un can cover females too sometimes, Then again..... I am not a mexican 
