Lionel is a medieval diminutive of Lion: literally "little lion," which makes it an unusually apt name for a large or impressively maned dog. There's a pleasant irony available too: a small dog named Lionel gets the full lion reference without pretension, because the diminutive is already built in.
The Lion's Diminutive Logic
Lionel is, etymologically, a modest name. That built-in diminutive makes it more versatile than Leo alone: it works on the enormous dog as ironic diminutive and on the small dog as genuine one. Chow Chows and Leonbergers wear the full irony beautifully.
The Messi Effect
Lionel Messi has given the name a global contemporary reference alongside its older associations (Lionel Richie, Lionel Barrymore). That sporting vitality adds warmth to a name that might otherwise feel purely historical. Football fans have an additional layer of affection available.
The Counter-Reading: Needs Some Gravity to Carry
Lionel works best on a dog with some presence: physical scale, an impressive coat, a self-possessed personality. On a nervous, fussy dog it sits awkwardly. The human name provides genuine crossover quality for owners who want a proper name rather than a pet label.