The pleasurable spreading was a wave of short, black glossy fur traveling from your crotch across your body. Wherever the fur touches also brings more changes to your body:
Reaching your feet, the fur becomes tan to outline your feet, causing them to stretch and form into hybrid dog feet/paws, allowing you to still stand bipedal. A soft, fleshy feeling forms at the paws, indicating paw pads growing in next.
Although obscured by the briefs, a patch of tan fur outlines your ass as it reaches it, causing your butt cheeks to seperate, exposing your pucker as it darkens. Your balls become enveloped by the smooth, black fur, causing the skin above it to thicken and stretch into a sheathe for your cock. Your cock also begins to change, becoming fleshier and bright red, tapering at the tip and swelling at the base, becoming a recognizable red rocket.
Next is your arms, tan fur again appearing from the black fur as it reaches your hands, outlining your wrists and palms. Along with the fur, your fingers and palms begin thickening until they resembled a hybrid mix between hands and paws as well. Like with your feet, soft dark paw pads push through your fingers and palms.
Finally, the fur reaches your neck, travelling up to your head, highlighting your lower mouth and neck with tan fur. With the fur, your nose and mouth begin pushing outward into the shape of a canine muzzle. Your tongue rolls out of your mouth, flatter than it used to be. Your nose shrinks in size but also thickens, becoming wet and leathery.
The fur rises up and over takes your entire face/muzzle, with two small tan fur patches 'replacing' your eyebrows closer to your eyes. Your pupils also begin dilating, causing your rounder eyes to appear darker, if not nearly a beady black. As fur travels up the sides of your head to your ears, the now familiar tan fur begins covering the inside of your ears. Soon they are travelling further up your head, becoming pointed and thinner.
Your head resembles a Doberman Pinscher now with some human shape as your hair recedes into your head, shortening into the same glossy black fur covering the rest of your body.
As soon as it finished, the rest of the short fur on your ear rushes into your head before you even get a chance to notice your changes, causing you to jump in surprise. A flood of new canine instincts, thoughts and memories rush into your mind, the shock turning into pleasure as any questions you had about the briefs and your new form were instantly answered unwavered by your new mind.
Looking down, your smile turns into a confident grin as your canine eyes come into contact with the package, now noticing the previously absent label on it, reading, 'The Dobie Pack'
If seen earlier, you would have insinuated it was the name of a specific line of Beast Briefs, but now you know it's true meaning: the bundle was to create his own pack of anthro Doberman to lead. And this mansion would be you and your pack's dwelling.
With a newfound goal you think you've always had and the warm squishing of your musky cum in your briefs, you pick up the rest of the briefs and think about your next course of action to build your pack.