Godrick is certainly not cutting the horns of that Omen. It isn't possible for him to re-grow new horns for him either. But I'm talking in a sense that he doesn't want to put him in the sewer, he found one and is fine with an omen working for him regardless.

Banished Knights aren't exact mercenaries though. As most of their locations are either Godwyn and Dragon related. (Castle Sol, Dragon Communion Temple) - you can also see displays of banished Knights armors in Leyndell so they were likely the elites.

Also based on Commander Niall armor.

"An old helm that has seen many battles.Worn by Niall, the great veteran general of Sol.

Nial was a lone survivor who commanded spirits to defend his long-passed master.

This aging general could not die, nor did he have anywhere to fade away."

Niall likely served Godwyn.