You are looking for trouble if you think you can sneek them in. If you claim the tarantulas are pets then you need an import licence to bring them into Thailand. By not doing so, you risk a great fine, destruction of your pets and trouble for you personally, in other words possible time in prison, cancellation of your visa and a quick exit out of Thailand never to return. Please respect Thailand and do the right thing.
GENERAL PROHIBITIONS AND RESTRICTIONS
The Thai Government controls the import and export of certain goods into/out of Thailand.
The controls either take the form of:
an absolute prohibition, which means that you are not allowed to import the goods in any circumstances; or
a restriction, where you need to have written permission in order to import the goods. The import of wild fauna, flora, fish and other aquatic fauna requires permission from the National Park, Wildlife and Plant Conservation Department, the Department of Agriculture, or the Department of Fishery as the case may be.