Hello!
I think the overall render looks fresh, appealing and efficient. Animation wise, the howling part seems spot on and really convincing!
I would have some suggestions/remarks:
The smallest one: maybe it’s intended, but the fur makes it look more like a plush.
Then, about some animation issues:
First, it needs a touch more follow trough, especially on the ears and tail.
Also, it’s clear that you forget to manage the Carpus and Hock joints (similar to our wrists and ankles) on some parts.
I would take as an example this frame:
With this kind of pose, it would be painful and uncomfortable to have this wrist angle, it should be kind of aligned with the arm and the paw. For the ankle, it seems relaxed and unbalanced while it should be tense en firm, I would bring it forward.
Fixins this joints issue will make the poses more “physically” correct and believable.
I don’t know for Umbreon, but I know for sure that Eevee is supposed to be kind of a dog, and dogs have a very precise gesture system.
The pose you made is near the “threatened” pose, dogs take this pose kind of as a dissuasion: “be aware, I grumble loudly, I have big teeth and if you do one more step I’m ready to jump at your throat!”.
You have the “be aware”, the “grumble” and the “big teeth” sides of the pose, but you’re missing the “I’m ready to jump” part, which IMO is very important and make the difference between other poses like “angry” or “attacking”.
Because the dog is ready to jump, he basically tenses up his muscles and posture for that: upper body lowered in order to give his arms a nice wide angle of action, the butt slightly behind his feet, ready to give a very strong hard push on his legs (and making the whole body go backwards, and the arms more extended and free for move).
Your pose is kind of in between all of that, the legs are extended far away, making them kind of “out of the game” for the eventual aggressivity engagement, the arms are not free.
I think that kind of attention to poses would make your animation look more awesome and emotionally intense.