Someone had a question on tripadvisor recently about Samui's wildlife...
What shall we be aware of? Snakes? Bugs? Mosquitos?
Basically the answer is - beware of all of them, but some more than others. Koh Samui is a rather developed island, so it really depends where you are. If you fancy a walk in the jungle you should be aware of the snakes and scorpions, but in the populated areas of Koh Samui your biggest worry is mosquitos, they are most common, you need to carry a good spray with you. Sometimes small mosquitos here can spread a dangerous disease called dengue-fever, which believe me, you don't want to get.
Personally I have never seen any snake here because I live and work in busy place, but my friend Ann who lives near jungle, sees them in her garden on a regular basis...A few months ago she took these pictures on her terrace:
The worst one for me is a spider. I am scared of big spiders to death, I don't care if they are harmful or harmless, I had it once in my bathroom, and I just never ever want to see it again.