Why do some summons have forehead protector while most of others don't?
Forehead Protector in Naruto shows the symbol of Ninja's hidden village and his loyalty towards it.

Similar can be assumed with Summoned animals, Summoned Animals with Summoned animals with  forehead protector seems to be loyal to one village only.

Even naruto.wikia.com speculated the same :


  Summons may also wear a forehead protector, suggesting a loyalty to
  only one village.