existing dog with new adopted old gsd
my adopted mix breed dog is coming to 2 this year
had recently adopted a old (unknown age) working gsd
first few days, the younger boy keep wanting to nip at the gsd ear, and the gsd didnt really bother with it.
past few days, the gsd is taking "action" on the young dog, he is saw a few times putting his paw on the younger dog back, i stop him and pull him away. young dog seen tail tuck between the legs.
sometimes, he is seen like trying to "hump" the young dog, which is smaller than him.
what is gsd trying to do? how should i manage the situation?