cockhorse

1. Something used as a toy horse, such as the knee of an adult or a rocking horse.
2. A horse added to a team of horses to assist a wagon through high water or over difficult terrain.

[Origin unknown.]

The noun has one meaning:

Meaning #1: a child's plaything consisting of an imitation horsehead on one end of a stick; the child straddles it and pretends to ride
  Synonyms: hobby, hobbyhorse, rocking horse, stick horse