A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity, i.e., constant viscosity independent of stress. In non-Newtonian fluids, viscosity can change when under force to either more liquid or more solid. Ketchup, for example, becomes runnier when shaken and is thus a non-Newtonian fluid. (Source)
I'm trying to make an artificial species that has tiny, subdermal sacs of non-Newtonian fluid that are designed to reduce injuries due to minor physical trauma; something like a punch or a baseball comes in, hits the skin, and these sacs suddenly change from liquid to solid to absorb a good chunk of the energy.
What are some good naturally generatable non-Newtonian fluids that I can use for this purpose?