Why do people write that hoeing off mares tails gets rid? it doesn't! It only breaks further down and comes back, double the trouble. You're pruning it! Horse tails go very deep and either you dig very very very deep when you first come onto the plot and remove it all, or you spot treat. I recommend Kurtail Evo. Spot treat, spot treat any new growth, and keep at it and it kills it. Take time to get rid and stop making the problem worse by pulling or hoeing it off.
While I'm being pedantic, mares tails (Hippurus vulgaris) grow in water or boggy conditions, horse tails (Equisetum arvense) grow on land.