I'm curious how you got them from your friend. I thought you couldn't trade fish and bugs.
Anyway, like most people, the tarantula and scorpion were by far the most difficult for me to catch. If I had to pick just one, it would be the scorpion, though, because I saw so very, very few of them. I spent hours on multiple days during their peak times just walking slowly around my town, waiting for them to spawn. It was enough to put me to sleep, especially with the calming night music playing. Then, when I would finally get one to spawn, it would disappear behind a tree or jump in a pond. So frustrating! LOL