I've tried many different pastel surfaces for pet portraits over the years but settled with Canson Mi-Teintes (I use the none textured side). When I start a new painting one of the most difficult decisions is the colour of the paper. The background colour acts as a ground and generally dictates the overall affect you'll get.
If I'm painting from a dog, horse or cat photo that dictates the background colour strongly then then I will paint on that colour. You can see this in the Black Labrador commissioned painting below which as you can see gives a dramatic but harmonised effect. I still however used black pastel but it means not so much pastel is needed and the paper does a lot of the work.
The Border Collie below had such a colourful background in the surroundings I took the photos in which was a meadow in the summer with bright blue sky - it just called out to me for a dark blue background, which it worked well and the recipient was very happy.
For most of my commissioned pet portraits unless the photo dictates a certain colour, I generally play it safe and go for a mid tone brown to add a bit of warmth and harmonise with the subject. The Grey Arab horse below worked on my faithful much used mid-tone brown because it adds a warmth and harmony to the painting which could easily become cold due to the horse being grey.