Defensive and Wall are not the same thing, but they are similar.
Usually, you can make a Pokemon either Defensive or Offensive based on its move set, ability, item, and EVs. In this case, a Defensive Arcanine uses status moves to help it play defensively. Intimidate is also a nice ability to help it switch into physical attacks. It can also wear down opponents with either passive damage (ex: Toxic / Will-O-Wisp) and attacks (ex: Flare Blitz). A Defensive Pokemon doesn't necessarily need to have high HP / Def / Sp. Def. Basically, a Defensive set is meant to survive a bit longer, but still dish out some attacks.
Unlike a Defensive Pokemon, a Wall is specifically meant for taking hits consistently over a battle as well as wear down the opponent by PP stalling or passive damage (note that it doesn't try to wear down opponents with direct attacks). It must have high HP and / or Defense and / or Sp. Def in order to perform its role successfully.
So, Defensive Pokemon and a Wall are alike, but they aren't the same.
Looking at your Arcanine move set, I would probably say that it is good enough. You could try using Toxic, but Will-O-Wisp is more important for dealing with physical attackers. I wouldn't try putting both moves on the same set because it would be a waste of a move slot. Roar is also nice for dealing with set up sweepers, but it isn't essential if you don't have any trouble with those sweepers.
Hope I helped!
