Idéfix made his first appearance in 1963 in "Asterix and the Banquet", where he followed Astérix and Obélix from a butcher's shop in Lutetia (which he was sitting in front of) all around Gaul. In the original French his name is Idéfix, itself a pun on the French expression 'idée fixe' (fixed idea) meaning an obsession. In English speaking countries the name Dogmatix is a pun on the words 'dog' and 'dogmatic'.
Idéfix is the only main character in the series that is an animal. His role is minor in most of the stories, significant mainly as a 'bone' of contention between Astérix and Obélix as to whether he should be allowed to accompany them on their adventures. However, he is often doing something interesting in the background and occasionally fulfils an important part of the plot.
In the words of the authors, Idéfix is the only known "canine ecologist" who loves trees and howls in distress whenever one is damaged. Despite his small size, he is quite fearless. He has drunk the magic potion on a number of occasions, but his favourite treat is to 'chew a bone'. Like many dogs, Idéfix is very protective and jealous of his master, especially when he falls for, or is shown affection by, beautiful young women.
-- wikipedia