Sorry, but I don't understand what this (Greek) sentence is trying to convey. Does it mean against in a physical sense, as in the dog is not only adjacent to me, but also leaning into or onto me (I am at least partially supporting its weight)? or is "against" used in a psychological/emotional sense, as in every time I come into the vicinity of this animal, it growls and bares its teeth, displaying fear or hostility?
I agree that this would be a very strange sentence in English. "Against" used this way implies an intentional thwarting of my plans or taking an opposite stand on a principle. This doesn't normally apply to dogs or other animals, even if they are growling or snarling at me. I don't think any of my neighbors would understand me if I uttered "this dog is against me." We might say, "this dog does not like me." in such a situation.
As a native speaker, I think it is a strange sentence in Greek too. It is not grammatically wrong, but Ok, never heard this phrase before about dogs. "Αυτός ο σκύλος δεν με συμπαθεί", "αυτός ο σκύλος δε με χωνεύει", "αυτός ο σκύλος δε με αγαπά", "αυτός ο σκύλος είναι εχθρικός απέναντι μου" and more, are normal to use in Greek in this case. I cannot explain in more, just my Greek language intuition and sense gives me this impression.
Εναντίον means against, versus, not just opposite. Opposite is αντίθετος or απέναντι (spacial meaning of opposite).