Well, yes. The possessive pronoun for the masculine case is indeed meiner. There are many tables on the Internet where you can look up the correct declension. Here's one of them.
However, note that the examples on that page do not contain a common noun.
In your example above, it's a possessive adjective, not a pronoun. There's not much difference; actually, most people confuse them. The important difference is just that a possessive adjective precedes the noun, the possessive pronoun, however, replaces the noun. This is true for any language (which contains such a thing), including German and English.1
In case of the possessive adjectives, there are some "exceptions" (they're not really exceptions as I will point out below) in respect to the declension. Again, there are many tables, and here's one of them.
Now, why it's not an exception:
When using the plain article you say
Da ist ein Koffer.
and you don't say:
*Da ist einer Koffer.
With that in mind, it's actually not a surprise, that the possessive adjective does not have an ending there.
1 To make the difference between possessive adjectives and pronouns more clear, I take elena's hints to elaborate on this:
In German, you can use the possessive adjective like this:
Das ist mein Koffer.
And if you replace "Koffer" with an possessive pronoun, you say:
Das ist meiner.
In English, there's also a difference between those two kinds of possessive 'indicators' (indicator here is not a proper grammatical term):
This is my suitcase. (-> possessive adjective/determiner)
This is mine. (-> possessive pronoun)
Here's a table on English possessive determiners and pronouns.