What about "the horse is very good"?
Why not "a very good horse"?
Because of the tha. That is the verb 'to be', so you need is in the English translation.
'A very good horse' > Each glè mhath.
Why mhath instead of math?
I have heard that "glè" changes the word after it so "math" becomes "mhath"
Horses are very good