Several theories regarding Brian's intelligence have been proposed. The version below was written by Alexander Pan of WhatCulture.
Family Guy has never really explained the reasons behind Brian's intelligence, his mannerisms, and how he is equally comfortable with licking his genitals one minute before immediately flipping a switch and enjoying a book with a nice glass of wine. Well, one prevailing fan theory is that Brian was actually once a human who had his brain inserted into a dog's body. It has been established that Family Guy, American Dad, and The Cleveland Show all occupy the same wacky universe. It has also been established that Klaus the goldfish in American Dad is actually a a German Olympic athlete who had his brain implanted into the body of a goldfish by the CIA. By connecting a few imaginary dots, fans have deduced that Brian must have also been subjected to the CIA's old brain switcharoo. This would explain the reason why Brian was the only one in his litter who acts like a human whilst the rest are just mere dogs. This crazy theory does have a flaw in that Brian isn't the only animal in the show with human characteristics.
Family Guy is actually a cartoon sitcom exploring a small Rhode Island town through the most haunting moments that come to shape its dark existence.
Around the 8th season of Family Guy, several characters' demeanors start seeing an incredibly dark change in character and tone.
Glen Quagmire, long known as a perverted comic relief character, starts to have his relationships with other characters flesh out. We start to see his broken family life beginning with his sister who is mercilessly beaten by her abusive boyfriend (S8E07 "Jerome is the New Black). This later comes as an emotional landmark to Quagmire where, in a later episode (S10E03 "Screams of Silence: The Story of Brenda Q."), he takes it upon himself to murder him. Later on in the series we see that Glen Quagmire's father, uncomfortable with hiding his sexuality, undergoes a sex change much to the dismay and disapproval of his son (S08E18 "Quagmire's Dad"). It's possible that a part of the reason for Glen's intolerance, is that his dad's repressed sexuality had a profound subconscious effect on a young Glen Quagmire. Not so coincidentally, the 8th season marks a point in time where Glen Quagmire's patience with Brian Griffin wears thin to the point of a full-blown feud between the two (S8E07 "Jerome is the New Black").
Brian Griffin, left in tears by an exhaustive rant of all of his flaws by Glen Quagmire, openly entertains the idea of suicide in the only episode without cutaway gags (S8E17 "Brian & Stewie"). Around this time he discovers that his writing may not be as hailed as it once was in previous seasons. An old, almost-forgotten TV pilot script of his does begin to take off until James Woods ruins the entire concept of his show (S8E15 "Brian Griffin's House of Payne"). It's possible that Brian, now aging and beginning to see decline in his writing, realizes that it was possibly his last chance to truly see such a dream come true. In S9E01 "And Then There Were Fewer" rival Glen Quagmire makes a public verbal jab at Brian suggesting that his broken dream was reason enough for Brian to murder James Woods. Perhaps Brian's broken dream was enough for him to consider taking a life; namely, his own.
Ironically, the darkest character at the beginning of the series, Stewie Griffin the homicidal super-genius baby with a penchant for Bond villain-esque gadgets, is growing to be one of the more noble characters of the series. He boldly steps out of his own way to rescue a depressed friend (S8E17 "Brian & Stewie") and longs to save a girl in a somewhat abusive relationship (S8E13 "Go Stewie Go"). It could be Stewie developing into a better person and has evolved into one of the lighter characters of the series. On the other hand, his plans for world domination aren't completely abandoned, his appetite for destruction is still not quenched, his gadgets are still present, and, until further notice, the plans to kill Lois are still underway. It may be that Stewie hasn't gotten lighter. The supporting cast has gotten darker. More likely, Stewie has become a symbol of just how messed up Spooner Street has gotten due to horrific events that have plagued them; the supporting cast has become just as, if not, darker than the once-unanimously darkest character of the series.
Alternate Possibility for the darkness of Family Guy: It's also noticable, incidentally, how as the rest of Family Guy got darker, it also coincided with Stewie's being Flanderized to become much more effeminate and using gay jokes. (Word of God's also claimed that Stewie is gay.) It's much less likely that, instead of the rest of Spooner Street becoming darker than Stewie came to terms with his sexuality, and as a result became a much happier person- completely incidental to the other cast of characters.
The deeply depressed cast of characters in Quahog includes former news anchor Diane Simmons who seeks murderous revenge on her co-anchor (S9E01 "And Then There Were Fewer"), Peter's boss Angela who threatens to commit suicide out of her crippling loneliness (S8E14 "Peter-assment"), new co-anchor Joyce Kinney who seeks bitter revenge against Lois Griffin (S9E09 "And I'm Joyce Kinney"), Bonnie Swanson who feels unsatisfied with her marriage and yearns to have an affair (S9E17 "Foreign Affairs", S10E23 "Internal Affairs"), recent widower Mort Goldman, divorced & departed Cleveland Brown, and local celebrity James Woods who has literally come back from the dead to ruin the lives of the Griffin family, among others.
The highly-strung nature of the Family Guy character cast and their increasing callousness toward one another points to a sort of uniform jadedness being experienced in all of Quahog. While the residents of Quahog had once featured a caring group of individuals in the community, it has become distorted into a place where people absolutely and publicly loathe each other, not even batting an eye when somebody of their own town is violently murdered. Quahog, Rhode Island is a poisonous town slowly suffocating its own inhabitants, turning them against one another, and threatening the health of any semblance of a traditional family.