Does the practice of not speaking directly about others' minds, but using ～がる to be indirect, apply to animals? i.e., would you ever say something like "犬は外に行きたがっている"? If so, does this vary when talking about your pets -- part of your household -- as opposed to unfamiliar animals?
The Dictionary of Basic Grammar only states that it can be used with a person other than the speaker. However, It seems you can use ～がる with animals. I don't think it matters if the animal is familiar or unfamiliar.
I found その犬は中へ入り[外へ出]たがっている. at Weblio.
So, it seems like the animal doesn't have to be familiar to you since in the second link its a news story.