Well, for starters, look at the edges of the picture. It's clearly just a picture pasted over top of another picture via a computer, and not very well done at that. Secondly, the fonts are all kind of blurry (even if I can't read more than a few Japanese characters, I can tell when they're blurry by looking at the edges). Thirdly, that HP is impossibly high for the set symbol used on the card (the set symbol being those double white stars to the bottom right of the picture), which is the set symbol for the Neo Genesis set. The highest HP on any card at the time this set was released was 120 HP. A fourth clue is that there was no Entei in the Neo Genesis set on either side of the Pacific. And a fifth clue is that there is no Entei card with that image as its card art. Also, aside from being blurry, some of the fonts just look off and/or faded. And the colorless energy symbols used in the Retreat Cost are incorrect as well, as the top spoke of the star appears too stubby. There may be more clues that I'm missing as well.
TL;DR: there's a lot that looks fake about it. And I do mean a lot.