"This pony is kind."
Translation:Tämä poni on kiltti.
Haha, I suppose. :D I guess if you are prone to talking to your pets etc. as you would to a child, then "mukava" doesn't sound that weird. But in general that's a good guideline.
When talking about an animal being "kiltti" it usually means "well behaved". Directly translated that would be "hyväkäytöksinen", which is also used. But "kiltti" is for sure the easier word.