1) When the adjective is in front of the noun it has to agree with gender, case, and number. "Wolf" in this sentence = masculine, nominative, singular.
2) The adjective is preceded by an indefinite article (an "ein" word), therefor it gets "mixed inflection" endings seen here: http://en.wikipedia.org/wiki/German_declension#Mixed_inflection.5B6.5D
3) If the sentence was "A wolf is red", the German would be "Ein Wolf ist rot" as "rot" is after the noun.
Hope that helps!