Blenny has been in there a long time 5 months,, this is the first I’ve ever witnessed this aggressive behavior even my blue devil damsel doesn’t mess with my ponies and I’ve seen the horses hitch to him and no problemYou really can't blame the Blenny though. if it was just tossed into the tank without researching to see if it was compatible with horses, then things like this are bound to happen.
Is the blenny actually attacking the seahorses? I've never known them to aggressive towards other fish.