@Brian855 While I'm sure it was disturbing to see that, if guests were to receive discounts every time they saw an insect or animal that freaked them out, hosts would go broke. Where I live in the tropics, there are all kinds of insects, some that could freak out guests, I'm sure. But they are endemic to the area, and not much anyone can do about it, except keep the inside of the house as clean as possible. Even then, there's no way to keep the occasional cockroach or ant from finding its way into the house.
If there were rats inside the house, that would be a different story, but people can't do much about rats in the area, and of course rats are drawn to water. I had year-long lease renters in my home in Canada who complained about rats in the house (I of course offered to send out the fumigators), but in fact, there was a widespread rat problem in the whole town, it wasn't limited to my house or the renters being dirty.
It doesn't seem to be the fault of the home owner- it was an outside drain.
Getting a discount isn't going to erase the fright you had. I'm sure that guests would appreciate hosts not complaining about every little thing the guest may have done that the host didn't like. For home stays to work, I think guests and hosts need to cut each other a bit of slack, especially when it doesn't have anything to do with the people involved being inattentive or uncaring. Just because the host was somewhat nonchalant about it doesn't mean he doesn't care that his guests are comfortable. It just means he's not as freaked out about seeing a rat somewhere outside as you are.
You could ask the host if he could please get or make some kind of cover for that drain ASAP so that water can still flow, but no critters can emerge, letting him know you're concerned about it happening again.
My cat deposited a dead rat on my outside terrace the other night. It definitely gave me a fright, as it was dark and I almost stepped on it. But I'm glad she's doing her job.
If my guests got upset about something beyond my control, I'd probably buy them a bottle of wine or box of chocolates or something, but all hosts may not think to do that.