Onion-skinning is an animation term that refers to making a drawing, then adding the next frame with the previous frame for you to trace from.
It's a litlle like making a flipbook.
Maybe you would like to create a physical flipbook. Watch the video below to get an idea of how onion-skinning works.
A collection of sprite sheets that you could use to help you with your animation.
Have a quick play with FlipAnim to get the idea of "onion-skinning".
If you want to do something more ambitious, download Pencil 2D and start animating. Mobile users can try Flip-a-Clip.
Flare, Synfig and DragonBones are professional animation tools and although Krita is an open source graphics editor a lot like Photoshop, it also has the ability to do onion-skinning animations.
Professional animation tool
Synfig is a professional animation tool.
Lovely free animation tool for Android or iOS.
Wonderful image editor
Open-source animation tool for Mac and Windows
Lovely desktop animation tool with onion skinning.