I have 19 equines; 2 saddle horses, 15 minis and a mini donkey. I have 4 mares that for various reasons are thinner than they should be, the other day they all got blankets. Yesterday, the donkey was shivering, so she got a blanket (she's coming 11 years old, and this is like the third time in her life she has worn a blanket, the first was during the Polar Vortex; evidently her porky little behind needs a longer blanket, as it's sticking out a bit). So far, the pushing 30 mini is fine, he definitely lets me know when he's too cold. And, everyone else seems fine just stuffing their faces full of hay.
Today our current temp is 27 below F with little to no wind.