Inulin vs. insulin
Inulin is a sugar, insulin is a hormone.
It's really very straightforward.
Inulin is a sugar which we can't metabolize, so bacteria etc. in our colon will do so, which means we get gas, bloating and so on, if we ingest enough inulin.
It might be good for diabetics, but if so that's just because it replaces some simple carb or sugar in their diet which then does not go on to become blood sugar, with all the problems that entails in diabetics.
You'll find it in the roots of some Asteraceae. Dunno if it's found elsewhere as well, but look for it in elecampane (Inula helenium), dandelion (Taraxacum spp.), burdock (Arctium spp.), and Jerusalem artichoke (Helianthus tuberosus), among others.
The name inulin comes from Inula, elecampane, as this particular sugar was first isolated in the root of that plant. No wonder, as a good elecampane root contains about 40 % inulin, in autumn, when it's at its sweetest.
Insulin on the other hand is a hormone secreted in the isles of langerhans in the pancreas. Insulin regulates blood sugar levels.
You won't find insulin in plants at all. Insulin used to be isolated and purified from animal sources (think sheep), but I gather that bacteria do the manufacturing job these days, for the insulin injections of type I diabetics.
The name insulin comes from insular ... it's from the islets of Langerhans.