Spastic Horse

Horse (If you control an untapped creature with the ability "rider" you may combine them
until end of combat into one creature that has the
combined power and toughness, abilities and creature
types of both creatures. If the combined creature would
leave the battlefield, both creature cards go the place the
combined creature would go. A combined creature's converted mana cost is 0)
As long as Spastic Horse has a rider it has +1/+1,
trample, can't block and must attack each turn if
able"