Video games are considered canonunless they contradict content in some other Forgotten Realms publication.
Boo was the pet and companion of the RashemirangerMinsc, having joined him shortly after he sustained an injury to his head.
While there was skepticism regarding the claim, Minsc insisted that Boo was, in fact, a miniature giant space hamster who was intelligent and could converse coherently with him. However, as Minsc was quite addled, it was unclear whether or not his claims were true.
In the Baldur's Gate series, Boo appears as an item in Minsc's inventory but if the player tries to remove him he scurries away and the protagonist receives a harsh glare from Minsc. He also features in Minsc's portrait in both Baldur's Gate I and II, but in the second game only his hamster squeaks are heard alongside Minsc's dialogue.
In the games Mass Effect 2 and 3, also from BioWare, the player can buy a "Space Hamster" in the human shop on the Citadel. In the Citadel DLC, Shepard tells the hamster to "go for the eyes" if anyone attacks it—something Minsc frequently says to Boo in Baldur's Gate 1 & 2.