Neuroglia evolved after neurons did. The evolution of glial cells coincided with increased complexity of the organization of the nervous system.
Glial cells are supportive cells for neural tissue. As such, evolution of glia before neurons would not be a parsimonious hypothesis, however not impossible.
Glia indeed seems to have evolved ...
Your question falls into an area of study known as allometry.
It is generally believed that brain size must increase as body size increases1,2. This relationship has been demonstrated in fish 3,4, including carp. This phenomenon is also seen in other (non-mammalian) vertebrates including the Nile Crocodile (Crocodylus niloticus)5.
This continued growth is ...