Fine, it isnt perfect now, but your hint was good.
I changed the line for the pet frame into:
--PetBar
CreatePanel("aPanels_PetBar01",0,0,218,31,background,border,"BOTTOMRIGHT","TOPRIGHT","aPanels_MButtons01",BT4BarPetBar)
CreatePanel("aPanels_PetBar02",0,0,218,31,background,border,"BOTTOMRIGHT","TOPRIGHT","aPanels_MButtons01",BT4BarPetBar)
The Result: The Panel appears when i call a pet and disappears when i dismiss my pet, but the panel got the wrong size now :<
The Pet Bar in Bartender is scaled 0.65 - I think that's the reason why the panel scales down too? How can i fix this? :<