Lennie is the affectionate short form of Leonard or Lennard, meaning "lion-hearted" from the Germanic leon (lion) and hard (brave, strong). As a pet name it has a soft, approachable feel that the full Leonard lacks — the kind of name you'd give a large, gentle dog who looks intimidating but melts at belly rubs.
Of Mice and Men Association
John Steinbeck's Lennie Small — the enormous, gentle farmhand with a love for soft things — casts a long shadow. Owners naming a big, soft-coated dog Lennie are almost certainly nodding to that literary character. Bernese mountain dogs and great Pyrenees fit the Lennie register exactly: large, sweet, slightly lumbering.
Human-Name Crossover
The human name Lennie peaked in mid-20th century America and now reads as vintage. That retro quality is appealing to owners who lean toward old-fashioned pet names — a deliberate choice against the flood of Luna and Bella registrations.
The Counter-Reading: Somber Undertone
Steinbeck's Lennie ends tragically, which gives the name a bittersweet resonance some owners may want to avoid. For those who want the gentle-giant feel without the literary baggage, Lenny is a lighter alternative.