I recently learnt about how we can create animated tilesets in Unity using some of the files included in Unity’s “2D Extras” repository. I believe the repository’s been discontinued, but you can still access it (as well as versions of it for previous Unity versions) here. I’d personally recommend selecting the “2018.3” branch, as that has a fairly quick and easy to use Assets folder (that we can just download as a zip, unpack, then merge into the Assets folder of our Unity project).
Once you’ve imported this, you should be able to right click in your Project view and navigate to Create > Tiles > Animated Tile to create one of these nifty animated tiles.
With it selected in the Project window, take a look at the Inspector and increase the ‘Number of Animated Sprites’ to however many sprites will be used in the animation. You’ll then need to drag these sprites into the appropriate slots in the Inspector manually, and beneath these slots you can also set the minimum speed, maximum speed, start time, and collider type to your liking.
Once you’re ready, try dragging your animated sprite from your Project window over into one of your tile palettes in the Tile Palette window. You should now be able to start painting using this animated tile 😉
(As an extra tip, you can also select some tiles on one of your tilemaps in the scene, then look in the Inspector and change the colour and/or alpha values. This can give you some nice transparency effects, which works pretty well when creating waterfalls for instance)