Leopard: Leopards first appeared in the wilderness encounter tables in Supplement III, and they’re getting stats here for the first time. Their only remarkable feature is that they are more likely to gain surprise than other monsters, and they in turn are less likely to be surprised themselves.
Leprechaun: These figures of Irish folklore and breakfast cereal mascots were first introduced in The Strategic Review #3, but this is their first entry in an official rule book. They’ve changed surprisingly little since then, gaining a measly few extra hit points and an immunity to surprise due to their sharp hearing. They still have the same special abilities (invisibility, polymorphing non-living objects, creating illusions and ventriloquism), the same 80% magic resistance, and the same penchant for fucking with the PCs. One new tidbit is thrown out, that being the possibility that leprechauns are a mix of halflings and pixies. I can live with that explanation, provided nothing better comes along.
Leucrotta: This new monster is sort of a stag with the head of a badger, said to be so ugly that most creatures can’t bear to look at it. It has a rear attack that it can use when retreating, but of greater note is its ability to imitate human voices. Leucrottas are smart, chaotic evil, and they can speak the Common tongue, so this is an ability that can be used to its most damaging effect. Nothing is said about them being able to mimic specific voices, though; I would rule that out in most cases. There’s certainly the possibility that a leucrotta could learn to do so, but I wouldn’t use it as a standard feature.
Lich: These undead wizards first appeared in Supplement I. Their Armor Class has improved since then, from 3 to 0. There’s a slight trade-off, though, as their paralysing touch can now be avoided with a saving throw. In OD&D, the lowest level that a lich could be was 12th, but in AD&D that has been revised significantly upwards to 18th. They also specifically get the usual raft of undead immunities to things like sleep, charm, cold, electricity, etc., whereas before this was not stated outright.
In OD&D Supplement III liches were given psionic powers. Here there's a note saying 'see below' in regards to their psionics, but nothing is detailed in their description. I'm inclined to just treat them like regular magic-users in this regard.
Perhaps the most significant inclusion here is the first mention of a lich’s phylactery, a piece of jewellery that houses its soul. It is only mentioned very briefly here as one of the necessities for becoming a lich, but no detail is given on what it is or what it does. But in the future it becomes one of the core features of this monster.
Lion: Lions first appeared in the wilderness encounter tables in OD&D Vol. 2, with Mountain Lions and Spotted Lions appearing for the first time in Supplement III. All three get stats here for the first time, and there are no surprises to be had. It’s the usual great cat treatment – hard to surprise, can rake with back claws, etc. Mountain lions are smaller and weaker than regular lions, and spotted lions are the larger prehistoric variety.