The titular image
The symbolism of stealing horses to give to his own family makes Trond a kind of right-fighter, but also a rebel, not unlike folktales like Robin Hood. Trond feels that the rules of society are only as powerful as he agrees they are, so when he realizes that there are people in town who are so rich, they genuinely wouldn't even notice if he stole a few horses, he does it. This symbolizes his passion for his personal opinions about justice.
The archetypal broken father
What is it that the father symbolizes in general? Typically the patriarch is a force of traditional inheritance, because a father instills lessons in the lives of young boys. But, Trond's father was changed by WWII, and the damage done prevents him from utilizing his fathering instincts, so although Trond and his father love each other very much, Trond is the victim of emotional neglect, left to his own resources.
Trond as a widow
Perhaps Trond might be a nice archetypal iteration for the orphan and the widow. He certainly has distant relationships to his parent figures, who he doesn't even understand as a kid (because he was not actually in WWII). Then, when his wife dies tragically, the motif is complete. He is an archetype for loneliness, because his instincts are constantly reminding him that he is too alone, but he feels life has driven him to be alone.
The dead twin
When Lars is revealed to Trond, Trond feels a flashback of true horror to the time he was playing with a gun, and it went off, killing Lars's twin brother. The pain that he caused his community is a symbolic reminder to his difficult relationship with community, because he is broken enough to be a serious risk to other people's health and safety—hence the isolationism.
The memories with nature
A rambunctious boy patiently fishing is a suitable metaphor for peace with God. A boy hunting for rabbits is basically just a predator animal. Trond remembers back to these nearly-mythic memories with the full weight of all his life's suffering and pain. He feels at one with nature when he is alone with the woods, or the Fjord, so he mainly stuck to nature his whole life. Now, in his eminent dying days, he is comforted by that closeness to nature.
