Question

I battled Red's Blastoise then after it kept using Water Spout, it used Struggle.Is this a glitch or is it true/normal?

Answer 1

In B2W2, Red's Blastoise is holding a Choice Scarf. When you run out of PPs of a move and are choiced, you have no option left but to struggle (or to switch out).

Bulbapedia - Red

EDIT: if you're also asking why it knows Water Spout, that's just the moveset it was given; no particular reason.