Not bad, I think the coloring is very nice and you have a good way of making the flames.

My suggestions are watch your anatomy. The left front leg bends very awkwardly and is very short compared to the right one and the right one is bent too far back. I would straighten it and bring it forward a big so you could maybe even see part of the "shoulder". The hind leg is very wide, either that or I'm not seeing the line where it bends. I would thin it down quite a bit, unless he has a ton of muscle back there which would be a bit awkward in comparison with his front since it's a little slimmer.

Also watch how low the shoulders go. Right now it kind of looks like his left shoulder is out of joint because it's riding a bit too low from the spine.

I would suggest thinning out the tail near the base a little so you can kind of see where it's coming out of. right now it looks like the whole thing is just attached to his bum and has no real start point. Or make sure the fur curves so it kind of hints that it's starting at one particular point.

I think that's about all I had to say, It's a nice drawing it just has a few anatomy kinks. The coloring, as I said, is quite nice and I really like that flame effect you have going. Oh and the head is very nice drawn.

Arcanine in your style, what can I say about it?
Yeah, you drew one of my favourite Pokemon wonderfuly.
I like the dynamic pose and its expression, how Arcanine looks ready to fight.
The simple background is cool aswell, specialy the fire. The shadding in the fur is soft and pretty.
The only thing I have to point out, though, is the left front paw. Where there is the articulation, it is too thin, specialy if you compared with the back paws or even the rest of this left front leg.
But really, this excepted, I think it's a good fanart. Nice job <3