Sylva, shapeshifted

Mounted creature gets +2/+2 and has horsemanship.
Whenever this creature leaves the battlefield transform it instead, then get a -1/-1 counter.
Mount {g/w} ({g/w}: Attach to target unmounted creature
you control. Mount only as a sorcery. This card enters the battlefield unattached and stays on the battlefield if the creature leaves. It's still a creature when attached. Attached creature can't attack or block, and its activated abilities other than mount can't be activated.)