I don't see how you could use subdomain.com by itself.
Are you saying you are forwarding a full domain like dogs.com to your subdomain at dogs.yoursite.com?
If so then I'm not absolutely certain but I believe you'll only get
PR benefit if you link to dogs.yoursite.com directly. I don't think the
PR benefit would pass through the URL forwarding.
However you need to weigh the visitor benefit too. If you think more people would click through to dogs.com then that's worth something too.