Here is the full list of all 27 Bibi & Tina - there are Adventures with Horses achievements.
Complete the practice lap
Complete the mission "Gallop to the Old Oak".
Complete the mission "Get to the castle as quickly as you can".
Complete the mission "Over hill and dale".
Complete the mission "Ride to Martinshof".
Complete the mission "Steeplechase".
Complete the mission "Gallop to the castle".
Complete the mission "Race to Crabman's stud farm".
Complete the mission "Ride out to the castle".
Complete the mission "Horse race to the ruins".
Complete the mission "Through woods and meadows".
Complete the mission "Ride out to the quarry".
Complete the mission "Cross country through the fields".
Complete the mission "To the old mill as quickly as you can".
Complete the mission "Downriver".
Complete the mission "Cross country against Sophia".
Complete the mission "Falkenstein lap".
Complete the mission "Off to the Falkenstein race".
Win the Falkenstein race.
Win the Falkenstein race ten times.
Groom a horse.
Clean a horse's hooves.
Feed a horse.
Complete 6 missions in under a minute each.
Ride out with Bibi ten times.
Ride out with Tina ten times.
Collect all collectibles.