Raymond Goodwin, 35, a quarry worker, committed 'an act of barbaric savagery', Mr Justice Igor Judge said.
Finding Goodwin not guilty of murder, the jury at Nottingham Crown Court accepted he had been provoked over the death of his Jack Russell dog, Charlie, and had suffered 16 years of cruelty at the hands of his father Eric, 73. The judge said it was his duty to accept the verdict, but he regarded it as a serious case.
During the three-day hearing, the jury heard that Raymond Goodwin went to his father's house in Wirksworth, near Matlock, Derbyshire, and stabbed him through the body with a poker. He then fetched his father's long-handled axe and beheaded him as he lay across the fireplace.
Blood-spattered and stripped to the waist, he carried the head in a plastic bag to show his sister, Linda Gregory, 42, and her husband Leslie, who lived next door. His sister's 15-year-old son, Dean, spotted him and shouted: 'Raymond's got grandad's head.'
He banged the head on the kitchen table and told the horrified family: 'I have sorted it out. He will never hurt me or Linda ever again.' Then he showed the head to another neighbour and was still holding it when the police arrived.
On the way to Chesterfield police station, he demanded the head back and told officers: 'It's my trophy.'
He claimed he was angry after being forced to drown his pet dog in the bath because his father constantly complained about the barking. His sister told the court: 'He loved his dog and would do anything for it.'
The judge told Goodwin his father was a 'violent, dictatorial man and you and his family suffered from that violence. But by the time he was 72, he faced the consequences of his behaviour in the form of a lonely old age.
'You were not wanted in his house, but you went back there. After killing him you went and found that axe. What followed was an act of barbaric savagery. You decapitated your father, then picked up his head and walked out of the house carrying it as a trophy, gloating over what you had done.'
Peter Joyce QC, for the defence, said Eric Goodwin ill- treated his family for many years and had beaten his wife Hannah even when she was sick and dying.
He told the court: 'Raymond is not a risk to the public. He is a lonely figure and had a particular affection for his dog, which caused so much trouble between him and his father. But he is very unlikely to hurt anyone else.'