var client = new MongoClient(connectionString);

BsonClassMap.RegisterClassMap<Animal>(map =>
{
    map.AutoMap();
    map.AddKnownType(typeof(Dog));
    map.AddKnownType(typeof(Cat));
});

var collection = client.GetDatabase("test").GetCollection<Animal>("animals");

collection.InsertMany(new Animal[] { new Dog(), new Dog(), new Cat() });

var dogs = collection.AsQueryable().Where(animal => animal is Dog).ToList();

Debug.Assert(dogs.Count == 2);

public class Animal
{
    public ObjectId Id { get; set; }
}

public class Dog : Animal { }

public class Cat : Animal { }

MongoDB.Driver.Linq.ExpressionNotSupportedException
  HResult=0x80131515
  Message=Expression not supported: (animal Is Dog) because is operator is not supported with the configured discriminator convention.
  Source=MongoDB.Driver
  StackTrace:
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToAggregationExpressionTranslators.TypeIsExpressionToAggregationExpressionTranslator.Translate(TranslationContext context, TypeBinaryExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateUsingAggregationOperators(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.Translate(TranslationContext context, Expression expression, Boolean exprOk)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateLambda(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.WhereMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider`1 provider, Expression expression, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.Execute()
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)