The owner of a dog killed during a suspected burglary attempt says the dog was her "whole world" and helped her survive night terrors and a degenerative nerve condition.

Police believe the two-year-old dog named Amigo was killed in the backyard of the owner's home in Melbourne's west during an attempted burglary on Tuesday afternoon.

Tracy Kenwery returned from a medical appointment to her Altona home and found her bull arab cross great dane dead near his bed with head injuries.

Ms Kenwery said the 50-kilogram dog wouldn't have attacked an intruder and described him as a "gentle giant".

"He would greet them, [he's] friendly, he'd go up for a pat, he just doesn't know anything different, there's nothing aggressive in him at all," she said.

Detective Senior Constable Joshua Curwood said it appeared the intruder had entered the backyard via the side gate and accessed the shed, before possibly being confronted by the dog.

"It could potentially be that the sheer size of the dog has intimidated whoever was walking through that yard at the time," he said.

Amigo was rescued by Tracy Kenwery from a rescue home and rehabilitated. ( Supplied: Tracy Kenwery )

'I'm still in shock': owner

Ms Kenwery said she rescued Amigo from a rescue home when he was just a few months old and still couldn't comprehend he was no longer by her side.

"He was my whole world, he went everywhere with me that I could take him and he slept with me," she said.

"I suffer night traumas, he wakes me from my night traumas.

"He is used to me putting my weight on him if I need to stand, things like that, so he's more than just a dog, more than just a family pet, he was very much a big part of my family and my companion.

"He's got three cats at home who are missing him greatly, they were prowling all around the house last night looking for him, they all get on really well.

"I'm still in shock. I even slept last night with his favourite toy, it hasn't sunk in yet."

Ms Kenwery said she was preparing Amigo to be her companion dog.

"I rescued him at three and a half months old," she said.

"He was taken from a place that had dog fighting. He already had a broken ear and scars on his face.

"I took him in from there. He was malnourished, he'd never been out of a cage. He was frightened of grass and we brought him up."

Police said nothing had been taken from the backyard or shed and there was no sign of entry to the home.