You can unlock RC vehicles to use in co-op mode by completing omochao's missions or by saving animals. For the first two vehicles in each category, clearing an additional mission in the how to unlock column will increase the vehicle's level. For the last three vehicles in each category, their level will increase by saving additional animals.

Vehicle	How to unlock
Balloon 1	Complete mission 24, 56, 59, or 97.
Balloon 2	Complete mission 32, 51, 64, or 81.
Balloon 3	Save 13,000 animals.
Balloon 4	Save 19,000 animals.
Balloon 5	Save 25,000 animals.
Fighter Jet 1	Complete mission 2, 14, 41, or 45.
Fighter Jet 2	Complete mission 9, 47 ,80, or 95.
Fighter Jet 3	Save 10,000 animals.
Fighter Jet 4	Save 16,000 animals.
Fighter Jet 5	Save 22,000 animals.
Helicopter 1	Available at start. Complete mission 4, 42, or 84 for upgrade.
Helicopter 2	Complete mission 15, 38, 53, or 71.
Helicopter 3	Save 9000 animals.
Helicopter 4	Save 15,000 animals.
Helicopter 5	Save 21,000 animals.
Hovercraft 1	Complete mission 29, 49, 72, or 94.
Hovercraft 2	Complete mission 3, 31, 73, or 98.
Hovercraft 3	Save 12,000 animals
Hovercraft 4	Save 18,000 animals.
Hovercraft 5	Save 24,000 animals.
Stealth Jet 1	Complete mission 46, 50, 91, or 99.
Stealth Jet 2	Complete mission 6, 54, 57, or 75.
Stealth Jet 3	Save 11,000 animals.
Stealth Jet 4	Save 17,000 animals.
Stealth Jet 5	Save 23,000 animals.
UFO 1	Complete mission 18, 30, 86,or 93.
UFO 2	Complete mission 37, 74, 96, or 100.
UFO 3	Save 14,000 animals.
UFO 4	Save 20,000 animals.
UFO 5	Save 26,000 animals.