Executing the 'getAnimalNameById' method on 'AnimalLocator' failed. Make sure the method exists with the name 'getAnimalNameById', is public and static, accepts an Integer and returns a String
Here are my codes:
.AnimalLocator-

public class AnimalLocator {
    public static String getAnimalNameById(Integer id) {
        String animalName;
        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);
        if(response.getStatusCode()==200){
            Map<String, Object> r = (Map<String, Object>)
                JSON.deserializeUntyped(response.getBody());
            Map<String, Object> animal = (Map<String, Object>)r.get('animal');
            animalName = string.valueOf(animal.get('name'));
        }
        return animalName;
   }
}

AnimalLocatorMock

@IsTest
global class AnimalLocatorMock implements HttpCalloutMock {
    
    global HTTPresponse respond(HTTPrequest request) {
        Httpresponse response = new Httpresponse();
        response.setStatusCode(200);
        
        response.setBody('{"animal":{"id":1,"name":"chicken","eats":"chicken food","says":"cluck cluck"}}');
        return response;
    }

}

AnimalLocatorTest-

@IsTest
public class AnimalLocatorTest {
    @isTest
    public static void testAnimalLocator() {
        Test.setMock(HttpCalloutMock.class, new AnimalLocatorMock());

        String s =  AnimalLocator.getAnimalNameById(1);
        system.debug('string returned: ' + s);
    }

}