From claim + -er.

claimer (plural claimers)

1. A person who makes a claim; a claimant.
2. A pretender to a royal title.
3. A racehorse offered for sale before a race and delivered to the buyer afterwards.
4. A claiming race.
   • 2002, Paul D. Siegel, How to Own Winning Standardbred Racehorses, page 61:

     He did not have a proper claiming authorization on file, and contended that the trainer had not asked his permission to enter the horse in a claimer.