I'll set aside the considerations of ethics, and write a bit about the meaning of the ability scores.
I've not seen 5e spell this out, but you can derive it pretty easily.
The beast creature-type covers non-magical animals. Going through the monsters in the back of the PHB, we find that most beasts have an Intelligence score of 1 or 2. Smarter beasts (mostly dogs and cats) have Intelligence scores of 3, and the giant eagle alone has an intelligence score of 8.
Given that the giant eagle is a fantasy creature, I would feel comfortable giving a rule of thumb that what we typically refer to as animal intelligence falls in the range 1 - 3.
Human intelligence is a little trickier. It is clear that a score of 8 is somewhere within the realm of human intelligence, because a lot of player characters have that score (8 is the minimum possible with the point-buy system).
On page 13, a score of 12 is referred to as part of "a set of numbers that are above average and nearly equal."
To me, this implies that a modifier of +0 (10 and 11) is an "average" score. This is, however, likely influenced by my experience with past editions. Exactly where the line is between "a little dense" and "there's something not quite right with him" is undefined.
Also, don't forget the influence of the Wisdom score when determining a character's mental health.
In D&D 3.X
Animal intelligence is explicitly declared to be 1-2.
A 10-11 was considered an "average" ability score.