Can a dog be transgender?

No dogs are not transgender no dog goes out voluntarily and tries to change it's sexual appearance in order to facilitate sex. Animals have been known to have homosexual relations. They're animals they don't know any better

Possibly.      

Well that depends if the dog wants both cock in his mouth or pussi 

While it may be possible to have a dog whose brain developed in one direction for sexual identity while the body and genitals developed in the opposite direction, it would be rather impossible for us to know since they can't express themselves verbally to articulate their feelings of mismatch.  If a male dog presents himself to another male to be mounted, or a female dog keeps mounting other females, how could we know whether those dogs are transgender and just doing what dogs of that gender identity usually do, or they are simply homosexual?  (Yes, dogs can be homosexual.  I used to have two separate friends who had male dogs who were homosexual, or at least bisexual, and only ever presented themselves to other male dogs to be mounted.)  In any case it's better to just let dogs be who they are, and accept them with all the love, hugs, and kisses we have.

Since it is impossible to communicate with animals on any meaningful basis it's impossible to know.

No, being transgender is limited to humans I believe.

No that’s crazy, EVERY ONE KNOWS only scorpions and armadillos can be transgender!!! ... n cats