These insulins are analogs, created artificially rather than extracting them from animals or duplicating human insulin. Although they function like insulin, they have specific molecular differences designed to alter their action profile.
Insulin analogs (or analogues) are chemically-synthesized molecules that function similarly to insulin, but have alterations which change their speed of absorption or other properties. The basic "building block" of all analog insulins is the human insulin molecule, as it is to this where the modifications for speed or length are made.
We in the Pet Diabetes world are most familiar with Lantus and Levemir as ultra-slow acting insulin analogs, but much research also goes into ultra-fast-acting analogs such as Lispro.
When pets and people are experiencing diabetic emergencies such as DKA the rapid-acting Novolog, NovoRapid (aspart), Humalog (lispro) or (glulisine) Apidra may be used to bring the blood glucose levels down. Dr. Nelson of University of California-Davis said in his lecture at the Ohio State Endocrinology Symposium in 2006 that if the short-acting analog insulins have any role in feline and canine diabetes, it is not yet determined
Many people with diabetes opt for one of these three for their bolus insulin, whether they use Lantus or Levemir for their basal or not.