What is the default Web browser on Android devices?
Are there weird Web browser set by default that I should be aware of (to test my mobile Website on it)?
That depends on the device and version. The most common, particularly for less new devices is the Android Browser. Like Chrome, it is based on Webkit and identifies itself as:
Mozilla/5.0 (Linux; U; Android x.x; Build/xx) AppleWebKit/530.17 (KHTML like Gecko) Version/4.0 Mobile Safari 530/17
Most recent - at least those from Google - devices use Chrome for Android instead. Phones use the token Mobile in the user-agent strings while tablets do not:
Mozilla/5.0 (Linux; Android x.x; Build/xx) AppleWebKit/530.17 (KHTML, like Gecko) Chrome/xx.xx Safari/<WebKit Rev> Mozilla/5.0 (Linux; Android x.x; Build/xx) AppleWebKit/530.17 (KHTML, like Gecko) Chrome/xx.xx Mobile Safari/<WebKit Rev>
Androids use the Android Browser as well as a few others that they can download. You can test your website on using an Android VM on your local machine. While for iPhone, iPad testing I recommend you get yourself these devices as there is no VM's available for these devices and the online emulators are not the real deal.