When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.

If a Zendikon and the land it's enchanting are destroyed at the same time (due to Akroma's Vengeance, for example), the Zendikon's last ability will still trigger.

The enchanted permanent will be both a land and a creature and can be affected by anything that affects either a land or a creature.

If a Zendikon's last ability triggers, but the land card it refers to leaves the graveyard before it resolves, it will resolve but do nothing.

An attacking or blocking creature that stops being a creature is removed from combat. This can happen if a Zendikon enchanting an attacking or blocking creature leaves the battlefield, for example. The permanent that was removed from combat neither deals nor is dealt combat damage. Any attacking creature that the land creature was blocking remains blocked, however.

An ability that turns a land into a creature also sets that creature's power and toughness. If the land was already a creature, this will overwrite the previous effect that set its power and toughness. Effects that modify its power or toughness, such as the effects of Disfigure or Glorious Anthem, will continue to apply, no matter when they started to take effect. The same is true for counters that change its power or toughness (such as +1/+1 counters) and effects that switch its power and toughness.