Yep, that’s a US site. As you’ll often see me say, device selection does vary by region.
Having a PIR motion sensor which won’t be set off by a small dog or cat it’s just a matter of adjusting the sensitivity. Most will be set off by a larger dog in part because as heat sensors, the fact that the dog has a higher body temperature than a person makes it trickier to ignore.
Some sensors have a physical button setting on the device itself, which lets you choose low, medium or high sensitivity. Others have a parameter that can be set by the system.
By far the most configurable motion sensor available that works with smartthings (that is, that offers you the most choice in settings) is the Fibaro multisensor. This thing has a crazy high number of choices you can make, including for sensitivity.
They make several different models that look identical. To use it with smartthings you want the zwave one, not the HomeKit one. And as with all zwave devices, you have to get the one that exactly matches the frequency of your hub. For the UK or Europe, that will be the EU frequency, which starts with an 8. For North America, you want the frequency which starts with a 9.
You can see the exact frequencies by region by looking at the official Z wave alliance products site:
https://products.z-wavealliance.org/
The aeotec trisensor doesn’t have as many choices as the Fibaro, but it does let you set the sensitivity from 1 to 11, which will probably work unless the animal is right on the border of the sensitivity ranges.