A direct replica of live action reference will look good. But will it be great? May be not. Your job as a character animator compels you to think about reference, but copying frame-by-frame is a big mistake. Sure, your goal is realism, but that doesn’t mean you should ignore subtle exaggeration. It is this exaggeration that will take your character out of the realm of being rotoscoped. Your goal should be to exaggerate the movements in such a way that the character becomes exciting & memorable.
Designing a quadruped alien? First step is to research & observe how quadrupeds move. Grounding your fictional characters in real world characters is the key to get your audience to accept your character. Sometimes we see characters that look like real world animals but move nothing like them. However, your demoreel is no place to be pulling off stunts like that. If you have a horse running around like a lizard, chances are that the recruiter will just move on to the next demoreel.
Working to create realistic characters means you will need to have a higher level of complexity. Add some dirt to specific curves in a way that feels real. Overlap the actions to make sure they affect (and are affected) by each other. Have the courage to have a slightly less perfect pose or arc at specific moments. But make sure this ‘messiness’ is very subtle & present only when required. Try to add little details, secondary actions, tiny overlapping actions, muscles firing & relaxing, etc. on your creature.
Very often on a demoreel, we see a large creature coming along & doing something cartoonier than the previous few seconds & then return to big-heavy-creature mode. The middle of the shot will have faster movements or overly stylized poses in comparison to the rest of the scene. This must never happen. Anytime you are animating, always remember that consistency is a big part of what you are trying to show on your demoreel.