As far as I know, here are the different pets (I will try to get pictures later):
CATS:
Twinkles - (Pretty Parlour) Orange cat with dark stripes. I think I have 2 variations. I'll try to get some pictures if they're different.
Catnip - (US Brush Me Beautiful) White cat, green stripes
Snowball - (UK Pretty Parlour w/Kisscurl) White w/green stripes....possibly different plastic and shade than the US.
Snowball, or unknown name - (poss. Germany?) I have another Snowball/Catnip cat, but the plastic is a very different shade of white and the green is also a different shade. Not sure where it came from though but I've seen a couple around.
DOGS:
Brandy #1 - (US Show Stable) Brown with brown accents, red tongue. Has slightly slanted round eyes.
Brandy #2 - (Unknown Show Stable) Brown w/brown accents. Has vertical oval shaped eyes.
Brandy #3 - (Unknown Show Stable) Brown w/brown accents. Has comma shaped eyes.
Name Unknown - poss. Brandy #4 - (US Home Sweet Home) Yellow w/purple accents.
Name Unknown - poss. Snowball - (US Megan's Place) White with purple accents.
DUCKS:
Duck Soup - (Waterfall) Orange with yellow accents. Far as I've seen, this is the only version, but please correct me if I'm wrong - I would love for there to be variants!
DRAGONS:
Spike #1 - (Dream Castle) Pinkish purple with green accents. Half-lidded eyes (sleepy looking) This may be a faded version of Spike #2, but the colors are so different, I doubt it.
Spike #2 - (newer edition of Dream Castle??) More of a deep purple color w/ green accents. Half-lidded eyes.
Spike #3 - (Mail Order) Deep purple color w/ green accents. "Comic" eyes, with a much more alert and happy expression.
Sparks (pink - Pr. Ruby),
Prickles (green - Pr. Aquamarine),
Smokey (blue - Pr. Sapphire),
Fiery (red - Pr. Pearl),
Flash (yellow - Pr Amber),
Spiny (grey/lavender - Pr. Amethyst) -
These came with the UK releases of the Princess Ponies. They all have the "comic" eyes, with a much more alert and happy expression.
BUSHWOOLIES:
Chumster(yellow - Pr. Primrose),
Cheery(lavender - Pr. Serena),
Eager (pink - Pr. Royal Blue),
Friendly (light blue - Pr. Tiffany),
Wishful (purple - Pr Starburst),
Hugster(aqua/lavender - Pr. Sparkle)
These came with the US releases of Princess Ponies. I don't know if they were ever sold in the UK. The only "variations" I've found were very slight color differences in the paint (I think it was a Cheery), but IMO it wasn't enough to call it a true variant.