It's an "ALIEN" not an actual dog.In it's final film variant, it's an Alien/Dog hybrid. The A3 alien never completely mimicked an actual dog. Did it bark? Did it wag it's tail or lick itself? Does a dog shed it's actual skin, drop down from ceiling and grab people,run on 2 legs at times or have an inner mouth that strikes out?
The pose is meant to be visually interesting,not to capture the "exact" physical moment of a dog in full stride. It's more closely capturing how the rod puppet would/could be posed if running. The original molded parts can also only be finessed so much before you have to start changing the original design elements to make it fit a very specific pose.The puppets had some slight modifications so their movement would look better while being filmed.
The original mold of the body,legs,arms,feet and hands are cast in a superman flying pose as one piece. The tail is cast separately and completely straight.The tail tip is cast separately as well as the main part of the head, the original lower part of the jaw, the inner mouth/tongue.The carapace that covers the head is a separate vacuum formed piece. All the mouth tendons are sculpted not cast. The neck has to be partially sculpted to fit that pose.There is a lot of finishing work involved to get the original molded parts into that type of pose.
The P1/CP pose has more of a look of a dog begging at certain angles,which isn't screen accurate.There are also several details on it that are not specific to any one prop. A few examples, the teeth are not accurate to the original molds/rod puppet.The inner tongue/mouth isn't the same. The whole head is based on the full size fiberglass A3 piece, not the maquettes or puppets. There are different and more pronounced details on the head of the A3 fiberglass fullsize piece in comparison to the original 1/3 scale molds.
The paint color and the paint details don't closely match either version. The latest pictures of the paintwork look really off-putting.
(Picture courtesy of Newborn of a raw casting from the original molds,except the head)