There are numerous antagonists in Clive Barker's The Thief of Always. By definition, an antagonist is something (typically another character, something in nature, or society itself) which opposes or conflicts with the protagonist (the main character of the text). For this story, one could identify three different antagonists: nature (the coldness of February and the pond), Carna, and Mr. Hood.
Nature is depicted as being an antagonist in a couple of ways. First, Harvey describes the weather of February as "the great gray beast." Harvey feels trapped by the cold, and he wonders if he will ever make it to Easter. Here, the cold weather oppresses Harvey. Another aspect of nature which acts as an antagonist is the lake with the poisonous fish. Harvey wants to swim, but Mrs. Griffin warns him about the murky bottom and poisonous fish that live there. As Harvey and Wendell ponder the lake and swimming, Harvey questions why the lake is so disgusting (given that everything else is so beautiful). Therefore, the lake and the fish act as antagonists because they symbolize evil and the unknown. Harvey cannot figure out their place in the beautiful and magical world.
Carna, "a swooping beast," also acts as an antagonist. This flying beast wants to keep Harvey in the magical world, not the real world. Since Harvey wishes to try to return to the real world and Carna tries to keep him from doing so, Carna acts as an antagonist.
Finally, Mr. Hood acts as another antagonist. Mr. Hood's magical world seems to trap the souls of children. Harvey comes to understand the evil nature of Mr. Hood after escaping Carna and getting back to the real world. Time moves differently in both places. Harvey realizes, after he sees how his mother has aged, that "for every day he spent there a year had gone by here in the real world." Mr. Hood controls the magical world and most everything in it, and Harvey's desire to leave the world identifies Mr. Hood as another antagonist he (Harvey) must face.