The mock HTTPCallOuts available in Salesforce are below

1. StaticResourceCalloutMock
2. HttpCalloutMock

Test class for HTTPCallouts in Salesforce

Sample HTTPCallout Class:

public class AnimalLocator {  

    public static String getAnimalNameById(
        Integer id
    ) {  

        Http http = new Http();  
        HttpRequest request = new HttpRequest();  
        request.setEndpoint(
            'https://th-apex-http-callout.herokuapp.com/animals/'
            + id
        );  
        request.setMethod(
            'GET'
        );  
        HttpResponse response = http.send(
            request
        );  
        String strResp = '';  
        if (
            response.getStatusCode() == 200
        ) {  

        Map < String, Object > results = 
            (Map < String, Object >) JSON.deserializeUntyped(response.getBody());  
        Map < String, Object > animals = 
            (Map < String, Object >) results.get('animal');  
        strResp = String.valueof(
            animals.get(
                'name'
            )
        );  

        }  
        return strResp;  

    }    
} 

Sample HTTP Mock Callout Class:

@isTest  
global class AnimalLocatorMock implements HttpCalloutMock {  

    global HTTPResponse respond(
        HTTPRequest request
    ) {  

        HttpResponse response = new HttpResponse();  
        response.setHeader(
            'Content-Type', 
            'application/json'
        );  
        response.setBody(
            '{"animal": {"id":2, "name":"Test"}}'
        );  
        response.setStatusCode(
            200
        );  
        return response;   

    }  

}  

Test Class:

@isTest   
private class AnimalLocatorTest {  

    static testMethod void testPostCallout() {  

    Test.setMock(
        HttpCalloutMock.class, 
        new AnimalLocatorMock()
    );    
    String strResp = 
        AnimalLocator.getAnimalNameById(
            2
        );  

    }  
    
}