Is a horse colicing if he has trouble peeing?

Colic is not a diagnosis, it's just a symptom. The term colic simply means abdominal pain and can have many many causes. So it is possible for a horse who is also having trouble urinating to show abdominal pain. But it is much more likely for changes in urination to be caused by kidney or bladder issues including infections or stones or metabolic issues. It is also much more likely for "trouble peeing" to really be muscle pain causing a horse to stretch and appear to be trying to urinate. This action is commonly described when horses are "tying up" or having muscle cramping and pain. If a horse appears to be having difficulty urinating, a veterinary exam is advisable.