Hippo.cs

using ConsoleApp5;

internal class Programm
{

    public class Hippo
    {
        public const int _fangs = 2; // клыки
        public static string habitat { get; set; } // ареал

        public double weight { get; set; }

        private string degreeOfAgression = "high";

        public Hippo() 
        {
            Hippo hippo = new Hippo();
            Console.WriteLine("Создание объекта Hippo");
            habitat = "Africa";
            hippo.weight = 3.885; // тонн
        }

        public void Print()
        {
            Console.WriteLine($"Ареал:{habitat}  Вес:{weight}  Степень агрессии:{degreeOfAgression}");
        }
    }

    public struct Behemoth
    {
        public int _fangs = 2; // клыки
        public static string habitat { get; set; } // ареал

        public double weight { get; set; }

        private readonly string degreeOfAgression = "unknown";

        public Behemoth()
        {

        }

        public Behemoth(string habitat)
        {
            habitat = "Abyss";
        }
    } 

    static void Main(string[] args)
    {
        Hippo hippo = new Hippo();
        hippo.weight = 2.800; // тонн
        Hippo.habitat = "Africa";
        Hippo.habitat = "Abyss";

        Behemoth biblicalBehemoth = new Behemoth();
        biblicalBehemoth.weight = 0; // тонн
        //Convert.ToString(biblicalBehemoth.weight);
        if (biblicalBehemoth.weight == 0)
        {
            Console.WriteLine("weight:unknown");
        }
        Behemoth.habitat = "Abyss";
    }
}