using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

public class DatabaseAnimal {

    public int id { get; set; } 

    public string animal { get; set; }

    public string type { get; set; }

}

​

public class Animal {

    public Animal() {

        animalType = new List<AnimalType>();

    }

    public string animalDesc { get; set; }

    public List<AnimalType> animalType { get; set; }

}

​

public class AnimalType {

    public string typeDesc { get; set; }    

}

​

public class Program

{

    public static void Main()

    {

        var dbAnimal = DatabaseValues();

        var result = dbAnimal.GroupBy(a => a.animal)

            .Select(grp => new Animal {

                animalDesc = grp.Key,

                animalType = grp.Select(e => new AnimalType { typeDesc = e.type }).ToList()

            });

        Console.Write(JsonConvert.SerializeObject(result, Formatting.Indented));

    }

    private static IEnumerable<DatabaseAnimal> DatabaseValues() {

        yield return new DatabaseAnimal {

            id = 0,

            animal = "dog",

            type = "TypeA"

        };

        yield return new DatabaseAnimal {

            id = 1,

            animal = "cat",

            type = "TypeA"

        };

        yield return new DatabaseAnimal {

            id = 2,

            animal = "dog",

            type = "TypeB"

        };

        yield return new DatabaseAnimal {

            id = 3,

            animal = "cat",

            type = "TypeB"

        };

    }

}