Cu Sidhe (5e Creature)
From D&D Wiki
Medium fey (sidhe), any non-chaotic alignment
False Appearance. While the sidhe remains on all four, it is indistinguishable from a large, ordinary dog.
Little Bit Beastly. The sidhe is considered both a beast and a fey for any effect relating to its type.
Like their distant relatives, cu sidhes have adopted well in the Material Planes, disguising as an ordinary dog in order to blend in humanoid society. As are many creatures petted and domesticated by humanoids, especially humans, cu sidhes sometimes struggle to maintain their fey nature, but a large slice of juicy steak always render them into a sweet little puppy.