Also, just a guess here, but compared to animals, body language is something we royally suck at as humans, but if any of the 16 types is going to be somewhat decent at it, it's going to be STPs.
On top of that, most people either don't wanna go near an animal or wanna pet it or touch it and look at it = big no no for the animal. But if you sit quietly, without any expectations, and make little to no eyecontact (no staring!)..animals will be curious enough to check you out on their own.
And I'd say that ISTPs with their observing ways definitely do this, without even realizing it