Beyond the Black River by Robert E. Howard

Conan the Barbarian is employed by one of the civilized countries to help in it's push to claim lands from the primitive Picts. The Picts are not excited about the idea however. Old gods and mythical creatures are called up by the Pict witches to contest the invading army and Conan finds himself battling for his life amid the blood thirsty hordes that include saber-toothed tigers, 40 foot long venomous snakes and a demon from another dimension who is intent on crushing him. The huge dog Slasher makes an appearance here and distinguishes himself so well in a doomed battle to delay their forces that Conan openly praises his courage and pledges that 7 Pict heads will roll in his honor.