Out of the three legendary beasts Entei always seemed to be like the more serious guardian to me. So I figured why do Enteis need to be so serious? They had to be puppies at one point right? So here you go a Entei letting him self go and indulge in puppy activities. I'm sure he is chasing small pokemon, or a ball. Maybe a Voltorb!
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Greyfox
Loved his expression. xD
Great pic.