I don't know about the Canadian registry but for AHA here's the rule for ownership transfers:
REG 124. CHANGE IN RECORDED OWNERSHIP FOR HORSES WITH A STANDARD CERTIFICATE OF REGISTRATION
To transfer the recorded ownership of a registered horse which has a standard Certificate of Registration, the recorded owner must complete and sign the transfer portion of the Certificate of Registration. A transfer of recorded ownership may be recorded if the following requirements are met:
- The completed and signed Certificate of Registration has been sent to the AHA Registry;
- The horse was resident in the United States or Mexico on the date of sale indicated; and
- The transfer fee has been paid.
On the back of the registration certificate is a section for the "Seller" that says:
The seller listed below must complete this section.
NAME OF CURRENT OWNER
A. Date of Sale _____________________________
B. Contract Sale
Begining Date of Contract ____________________________
Ending Date of Contract _______________________________
There are 2 lines in case of multiple owners.
Then the Buyer's Section:
The Buyer (new recorded owner) must complete this section. Please enter your ownership number below. If you have never registered a purebred Arabian in your name, leave this line blank and an ownership number will be established for you.
And then follows several lines for Ownership Number, Ownership Name, Address, & phones, and then signatures.
As far as AHA is concerned, this is proof of ownership. A Bill of Sale is nice but not necessary. As a seller, I check fairly frequently to make sure all horses that I don't own are transferred out even if the new owner never transfers them. If you check one of those horses they show up as me being last owner but then it says, STATUS Owner no longer owns, and gives the date I reported it to the registry.
I could not call and tell them to cancel a transfer. Once those papers are signed and someone has paid the transfer fee, there is a new owner as witnessed by my signature and I no longer own that horse and hoave no right to cancel the transaction.