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The local guarantor is where you are currently applying for the visa. You can just put the hotel where you are staying. Often times if you use an "agent" you would just put their name there (or it would be pre-filled in). In some cases, you can even leave it blank.
The Guarantor in Thailand is just someone that you know you will be visiting in Thailand. It could also be the guesthouse or whatever of where you are staying.
I want to remind the readership that these 2 sections are the least important parts of the Thai Visa application and are nothing to lose any sleep over.