What you need to think about is the way in which where animals place themselves reflects either their character or their position of influence in the farm as a whole. Thus it is that, for example, the pigs, who of course come to be the leaders of the revolution and hold the most power, place themselves "in the straw immediately in front of the platform." More insignificant or less important animals, such as the hens, "perched themselves on the window-sills." Mollie, the flirting horse, also goes near the front, but this does not reflect her position of stature in the farm community as a whole, as the text makes clear:
She took a place near the front and began flirting her white mane, hoping to draw attention to the red ribbons it was plaited with.
Mollie therefore places herself near the front as a reflection of her character in the hope that she can be noticed and because of her flirtatious nature. The cat, too, places himself as a reflection of his character and his lackadaisical attitude towards what is going on.