I have a 250 outlet but need it to be able to support 220 plug for a horse treadmill. Any help?

250V is the rating of the plug. It's good to 250 V. Your service is typically 208 or 240 if in the US.

220 is dead in the US.

Here is a list of common plug configurations typically called NEMA XX-XX

NEMA Configurations (http://www.nooutage.com/nema_configurations.htm#NEMA%20Configurations)

The required circuit (amperage, wiresize, breaker) defines the outlet.