In words of my Chinese teachers, 个 is something like a universal measure word. In case you do not remember the measure word for animals or wathever else, you can use 个, but it is better to use the appropiate one for each word.
Lol. Under the tree. It sounds as if the dog were buried. There must be a better translation. I Think they mean the dog is next to the tree because the dog would be on the ground. Dogs don't climb trees.