Why Arcanine is so low on the Tier List.
This is my guess as to why Arcanine is in Tier 3 and not higher. I could be wrong but I doubt it.
Short answer: He's no longer 2nd in terms of neutral DPS. He is still very strong against Exeggutor and Lapras, but those two are #7 and #9 as far as Max CP goes, so you aren't going to see them defending much.
Long answer: Prior to the CP rebalancing, Arcanine had quite a few things going for him. He had the 2nd best DPS in the entire game, behind Dragonite, so for a lot of players he was their best option if they couldn't rustle up a good type counter. Also he had type advantage on Exeggutor and Lapras who were reasonably common in gyms. Basically Arcanine used to be the best option for Snorlax, Lapras, and Exeggutor for a lot of people. All 3 of those were in the top 6 CP pokemon so you saw them pretty often.
Now, post CP rebalance Arcanine no longer has the 2nd best DPS in the game. That hurt his ranking pretty badly, mostly because now there are a lot more good options for Snorlax. The other thing is... the top tier defenders he had a type advantage on are still as good as they've always been... but they don't look nearly as good and the pokemon that now look better are good against him.
There's a decent chunk of the playerbase that doesn't know that highest CP does not equal best defender. Lapras and Exeggutor now are at #7 and #9 for Max CP and BOTH ARE UNDER VAPOREON. With as common as Vaporeon is and with the game doing a terrible job of explaining defensive strength, you probably won't see Lapras or Exeggutor in gyms much.
Added to that is the fact that not too long ago the Gym meta basically said "Max CP is king" because level 10 Gyms never really fell, but they did get chipped at. That's changed now, but I'm not sure how many people have realized that with level 10 Gyms actually being killable, putting in good defenders and not just high CP cardboard is important.
To make it even worse... those people who do know that Lapras and Exeggutor are still better defenders than all of the buffed pokemon are the most likely to have a Snorlax to drop in.
Answers
Yeah, all you have to do is look at the gym composition to see how many defenders now have a type advantage vs. him.
Vaporeon and Gyarados are pretty much in every gym now.
So is Rhydon...and possibly Golem.
That only leaves Dragonite, Snorlax, Lapras, and Exeggutor. The 2 of them he was worst against to begin with (Dragon, Snorlax) got buffed, making him less valuable. The 2 he was best against (Lapras, Egg) both got nerfed, making them less common
You really think Exeggutor is a great defender? Personally since my Arcanine sucks (Bulldoze!) I just bring in one of my Flareon soldiers (got dozens of them, like everyone does) and as long as I dodge his special, my Heat Wave or FB brings him to ashes pretty quickly. I'm usually happy to see Eggs in the lineup because I know he'll be easy to toast. The main benefit from the gym perspective is it forces me to bring a Flareon, which are otherwise useless against the vapes, Snorlax etc.
