You need balls to stay on Entei.Okay.
Venusaur switches in on Slowbro's Toxic.
Venusaur uses Swords Dance. You switch to Entei. Entei is now at 81% health.
Venusaur does 76.7% - 90.5%. Entei dies most of the time. If it doesn't die, it uses Lava Plume and does 52.5% - 62.5%. Venusaur easily survives, kills you, and has enough health left to attack two more times, unless you burn it, in which case it just kills you.
So yeah, you have small chance of actually doing anything, and an even smaller chance of doing something useful. Not what I would call a "counter".
Compare this to, say, Arcanine, who does 108.3% - 128.2% with 0 ATK Flare Blitz, has Intimidate (so boosted Earthquake from Venusaur does "only" 67.7% - 79.7%), and has instant recovery in morning sun.
tl;dr don't use defensive Entei
Edit: And I don't want to seem like I'm against "gimmicks", since most of my teams involve novel and less-than-standard Pokemon/sets. But there's no point in using a Pokemon for something it sucks at or in something which another Pokemon does much better.
Physical Arcanine and CM Entei are entirely different pokemons. Sorry, but i just don't know what you're saying.
And, first: use it before you call something gimmick.