I think the stink lines are only if they’re extremely dirty? Like if the hygiene need is yellow (even if we can’t see it) the tooltip will start to say dirty, but if it reaches red the stink lines appear and the animal may threaten to run away. It’s similar to the chicken coop, that you can and should clean before it shows stink lines or all the chickens will be dirty and unhappy. (Or the sims themselves, who aren’t visibly dirty until hygiene is red) It’s just a daily chore to clean the animals and their homes.
Though I do like how the horse beds have multiple visible stages. It’s particularly helpful since you don’t always notice when/how often they actually get used.
