Analog insulins Novolog, NovoRapid (insulin aspart), Humalog (insulin lispro), and Apidra (insulin glulisine) are all considered fast/rapid acting insulins, genetically engineered or modified for faster absorption.
Each of these has had the natural amino acid sequence of human insulin altered in a different way. All three alterations have been designed to keep the insulin molecules separated. Normally, insulin molecules clump together in groups of six (hexamers). Finding ways to keep the hexamers from forming means the insulin will be absorbed more rapidly, thus working faster.
They have a faster onset and shorter activity profile than the short-acting class of R/neutral and semilente insulins. They're used for bolus to cover meals, for "corrective" purposes when blood glucose is too high and for diabetic emergencies.
These, like their short-acting cousins, are able to be given either by injection or intervenously. Fast-acting and short-acting insulins can also be referred to as prandial insulins, because their most common use is to cover the food eaten at meals. 
British National Formulary classes both fast/rapid-acting and short-acting insulins under the category of short-acting.
Dr. Nelson of University of California-Davis said in his lecture at the Ohio State Endocrinology Symposium in 2006 that if the fast-acting analog insulins have any role in feline and canine diabetes, it is not yet determined. 
Fast Acting Insulins
- ↑ Heinemann, Lutz (January 2008). Variability of Insulin Action:Does It Matter?-page 40 (4 of 9). Insulin Journal.
- ↑ Nelson, Richard (2006). Selecting an Insulin for Treating Diabetes Mellitus in Dogs & Cats-Page 39. OSU Endocrinology Symposium.