Just to emphasize the point the GrifTheGreat and Eel Noob made - if you are concerned, a soak in warm water will often help the frog defecate. This should be part of any horned frog "bag of tricks"... I have noticed that intervals between bowel movements can be highly individual - it is not unusual for my big female Cranwell's to go three weeks (or more!) between movements. I suspect that feeding schedule is part of this - she is fed only once (or sometimes twice) a week. I also find that some foods (hornworms, nightcrawlers, silkworms) are more conducive to BM's than harder to digest foods like Dubia and mice. This is one reason that nightcrawlers are my favorite food for horned frogs (plus their calcium to phosphorus ratio is great).

PS. I have made a weekly soak in a good depth of water part of my horned frog routine.
PPS. Ralph did a truly GIGANTIC poop last night while soaking ! I swear it was bigger than my male ornate, who's no shrimp.