I researched Prime Taurus and you don't need two level 15 dragons to breed it. You just need to breed black + yellow and it's a 16 hour breed time. I think they just made a mistake in naming him. It should be called Neo Taurus because it doesn't have the prime symbol above it.
Prime Taurus Dragon
Rarity: Ultra Rare
Type: Zodiac, Yellow
Habitat: Zodiac Zone, Yellow Plateau
Available at: Level 40
Buying Price: 1,200 Gold
Selling Price: 100 Coin
Breeding & Incubation: 16 hours
Exp Gained: 3,000 Exp
Availability:
The Prime Taurus Dragon is obtainable:
For a limited time during an unspecified window of availability.
By purchase at the market for 1,200 Gold.
By breeding a dragon that provides the Black type with a dragon that provides the Yellow type. Missing requirements cannot be fulfilled by the splitting of minor types; parents must have the originally required types without splitting.