Lens distortion fix for composite image

[PamD]

Hi, I'm in a pickle, I'm creating a composite image of my friends 7 horses as a wedding gift, one of the horses is out of country so another friend took a photo of him in the hopes I could add him in. Thankfully the lighting should work with the final composite but the distortion is troubling, in reality he is the same size/height as the black horses but even after playing around in lens correction filter, Adaptive wide angle and warp I can't get him to have similar proportions. Is this a lost cause? I've added the nearly final version plus a same sized horse at a similar angle (black horse), their heads should be the same size but when I reduce the distorted image so their heads are the same size he looks like a little pony. Any help is appreciated. 

[Damien Symonds]

It'll have to be something like this, won't it?

[PamD]

Thanks for the reply Damien, why does that work so much better than my attempts? I'll do some rearranging; this looks like it'll work. 

[Damien Symonds]

25 minutes ago, PamD said:

Thanks for the reply Damien, why does that work so much better than my attempts?

Because you didn't use the body from the spare black horse.

[PamD]

mind blown... nope I definitely didn't do that! Off to head swap a horse! Thanks!