* Mammals are warm-blooded animals that have fur or hair, give birth to live young, and nurse their offspring with milk.
Let's look at the other options:
* Turtle: Turtles are reptiles, which are cold-blooded and lay eggs.
* Butterfly: Butterflies are insects, which are invertebrates and don't have any of the defining characteristics of mammals.
* Fish: Fish are aquatic vertebrates, but they are cold-blooded and lay eggs.
Only horses fit the criteria for being mammals.