How come Pakkun can talk to Kakashi when summoned (in fact almost all summoned toads can talk), but Akamaru, who was brought up by Kiba since he was a pup, cannot speak?
Although both Pakkun and Akamaru are considered ninken (dogs with heightened senses and abilities, and as such, working alongside shinobis), the only difference between them is Pakkun is actually a Summon (much like sage toads). Summons are basically supernatural animal spirits and they take on anthropomorphic qualities like talking, using weapons, wearing clothes etc. Akamaru is basically a highly trained dog that can use ninjutsu.
Atleast, Akamaru tried talking.