This is an expected behavior because of how case class works. case class automatically extends two traits, namely Product and Serializable.
Product trait is extended as case class is an algebraic data type with product type.
Serializable trait is extended so that case class can be treated as a pure data – i.e capable of being serialized.
Unlike case class Dog and Cat, your trait Animal does not extend Product or Serializable. Hence the type signature you see.
When you declare something like Array(Dog(""), Cat("")), scalac needs to infer single top type that can represent all the elements of given array.
That’s why the inferred type is Product with Serializable with Animal as Animal did not extend Product nor Serializable while the case class did implicitly.
To work around this inference, you can either make type explicit by Animal or make Animal extend Product and Serializable.
trait Animal extends Product with Serializable
case class Dog(name: String) extends Animal
case class Cat(name: String) extends Animal
Array(Dog(""), Cat("")) // Array[Animal] = Array(Dog(), Cat())