Eohippus was the first KNOWN descendant of the horse we know today. It was about the size of our average day fox. It had four toes on it's front feet and three on the back.
In US English, because you are referring to the flock, which is singular, it would be : The flock of birds IS... In British English you would normally say: The flock of birds ARE ... Group words such...
Its the first species in the equidae family, so the first horse. Its was around 14inches high, about the size of a fox and had 4 toes on the front feet and 3 toes on the hind.