I did the same thing (put the green novice horse into a random mount instead of the blue starter horse) and am stuck in the exact same quest step. I opened a ticket at https://blessonline.zendesk.com for the issue. Their support team replied already and confirmed the bug and said its been escalated to the appropriate team.
Hopefully it will be fixed soon similar to the Repair Kit tutorial (now you can use any repair kit to complete that quest).