Having venerated animals is supposed to give your colonists a mood bonus depending on total body size of the venerated animals per colonist. The tooltip states that you should get +1 mood for a total of 1, +2 for 2, +3 for 4, +4 for 6, and +5 for unspecified "tons". However, the actual effect is always one stage higher than the tooltip says: +1 is skipped entirely, you get +2 for 1, +3 for 2, +4 for 4, and +5 for 6.
Here is a save file with easy reproduction: https://drive.google.com/file/d/1-TUW49winKK980LDpZ-Xyaeb2PoOokN4/view?usp=sharing
You can see that there is one colonist with an ideoligion that venerates rats and 10 tame rats (body size 0.2). This gives a total body size of 2 for 1 colonists, and should give +2 thought; however, the actual thought is +3. If you release one rat [1.8], the thought becomes +2. Continuing to release rats, +2 is maintained until there are 4 rats left [0.8], and the thought completely disappears.
I've got similar results with tame alpacas, which have body size 1, but rats show it more granularly.
Expected behaviour: the tooltip should match what is happening in the game. If the tooltip is intended to be correct, then mood boosts should be changed as appropriate, and the tooltip should state how many animals you need for +5 (right now it just says "tons of animals"). If the tooltip is wrong, then there should still be a +1 thought for having more than 0 but less than 1 total body size per colonist.
Thanks for the bug report and apologies for the inconvenience of it. Also thanks for the provided save, it made researching the issue very easy. I believe what is happening is something that has happened elsewhere where things start at 1 instead of 0, so you start with a higher bonus than intended. I'll be posting this thread and the save to the devs for their review.