Why "Häusern" and not "Häuser"
Why is there a "-n" suffix for word "Häusern" in the following sentence:
Mein Haus ist zwischen diesen Häusern.
According to several dictionaries, the plural of "der Haus" should be "die Häuser".
As Pepan said, it's because of the dative case. Certain prepositions trigger certain cases. "Zwischen" is a so-called "two-way preposition", which means that it is followed by either the dative case or the accusative case. The accusative case is used when there is motion ("I throw a ball between the houses") and the dative case is used when you're talking about a fixed position, as in your sentence ("My house is between these houses"). When you use the dative plural ("diesen Häusern"), you do not only change the article but also add an "-en" or "-n" to the noun, unless the plural of the noun ends in "n" or "s". That is always done and it is not old-fashioned or dated. The form you found in your dictionary, "die Häuser", is nominative plural. Most dictionaries only give the nominative singular ("das Haus"), the genitive singular ("des Hauses"), and the nominative plural ("die Häuser"). But if you like, you can also see all the forms here (enter the noun and then click on "word forms"): http://www.canoo.net/index_en.html
I believe it's because the dativ case. From time to time, the word changes depending on the case even when in plural form. Also I believe, that it's kind of an old-style German but is still used especially for basic words or in phrases.