Horse does not have to be registered to enter.....there is a paragraph in there somewhere about health certificates, coggins, etc. You enter through the regular premium book......the pre entry dates are on the entry form/rules but if I remember correctly it was January 10th maybe and online entry by Jan. 20th with no late fee....
The horse show office staff at San Antonio are very helpful, do not hestitate to call them, you can ask for Sara Motal, she is who I have worked real close with over the phone. All stalls will be reserved through the stock show as well. phone # 210-225-1309