After years of trying to catch both of the dreaded beasts (I'm more terrified of approaching them in City Folk
), I finally caught them last month. Not in the same night, of course. For the scorpion, I used a method called "The Bus Stop Method", meaning that you get the scorpion to chase you and once you get to the bus stop, run up it. It didn't follow me on, so I swung my net and caught it. The tarantula was just wandering around, so I slowly walked up to it. I honestly thought I was going to miss when I swung and get bit, but I didn't.
Here's a video if anyone needs help: Tarantula and Scorpion Catching