Rabbit fag here. Even the most domesticated buns would know the fox is there and be alerted. Buns will likely smell the fox first and their sense of smell is how they primarily interact with the world. The bun is also staring at the fox out of its left eye. Bun eyes are on the sides of their heads and they only have a small field of binocular vision in front and have a pretty bad blind spot particularly up close. Rabbit vision is far sighted so they can see threats and run from them early. I also think that bun is in the process of thumping to alert others of the danger, you can see his right back leg is oddly positioned, normally it would be under the body and not visible. For being fat it's just a dewlap, we did that to them just like the lop ears. The lop ears also make it harder to judge what the rabbit is feeling.