Simba
MP required to summon: 2 MP
Attack: Proud Roar
Obtain from: Talk to Leon (Squall) in the Secret Waterway in Traverse Town to get the summon gem Earthshine.

Bambi
MP required to summon: 1 MP
Attack: No attack, Bambi runs around you, dropping MP balls and items.
Obtain from: You get the summon gem Naturespark after beating Pooh's Hunny hunt. The Torn Page will turn into this item.

Dumbo
MP required to summon: 3 MP
Attack: Splash
Obtain from: In Monstro, there is a chest near the front of the mouth on a high platform. Hi-Jump/Glide up there. Open the chest to obtain Watergleam.

Mushu
MP required to summon: 3 MP
Attack: Fire Breath
Obtain from: Beat Maleficent (Dragon form) to obtain Fireglow.

Genie
MP required to summon: 2 MP
Attack: Showtime
Obtain from: Seal the Agrabah Keyhole and Aladdin will ask Genie to go with you on your quest.

Tinkerbell
MP required to summon: 3 MP
Attack: Cure/Revive. Tinkerbell will cast Cure on you and your party members in a rotation. Also, if Sora is K.O'd, she revives him once, but then the summon is dismissed.
Obtain from: Seal the Neverland Keyhole and Peter Pan will tell you to take care of Tinkerbell for him.