Aw, I like Frog the Dog!

But then I think all dogs are cool!
Anyway, Frog's history begins in the 5th. Season episode of MacGyver called "The Black Corsage". He is initially owned by Sophia Ross, a Bulgarian scientist who is aided by the Phoenix Foundation and at the end of the episode she gives Frog to Frank Colton, who was also helping MacGyver.
I believe Frog mainly shows up from there in the 6th. Season episode "Squeeze Play" and the 7th. Season episode "The Coltons".
I don't think he's shown up in any other episodes beyond that- though maybe he was seen in the background in some of the other episodes featuring The Coltons- though as far as I can remember I don't think so.
Frog's most notable moments probably include helping free Frank Colton and MacGyver in "The Black Corsage" by bringing a fish with sharp teeth to MacGyver- and later in the prologue of "The Coltons" episode by bringing a bottle of acid to MacGyver so he could cut the ropes binding him and Frank.
Bulldogs make great watch dogs and are definitely good for protection- as Mama Colton cautioned Billy to take Frog with him when he went after Jesse and Frank in "The Coltons" episode.