What is transsexual with dog?

This is difficult to answer not knowing the context. If you read it in a local personals page, it likely means the person has transsexualism and owns a dog. When the term transsexual is used correctly, it refers to a person who was born with transsexualism and who will eventually have surgery.

However, pornographers use the term transsexual to refer to crossdressed men (or non-op, male-born transgender individuals, and in rare cases, actual pre-op TS persons who allow themselves to be exploited in such a manner for surgery money), who model things being done with their genitals. Most are not transsexual persons but give actual transsexual persons who need surgery and the chance to live as normal women a bad name. So if you are asking in the context of such hard core pornography, I am not sure you really want to know, since the term "with" might imply sex. So in the context of hard core porn, with the term transsexual misused, specifying they are "with" an animal may suggest illegal sex between the two.