Wheelock introduces 'Laocoon, -ontis (m)' as the character in a story. However in the text, the form 'Laocoonta' appears. The -is genitive ending means that it is 3rd declension no? With endings in -em, -is, -i, -e, -es, -um and -ibus. So how can a masculine 3rd decl noun end in '-a'?
The so-called Greek declension crops up when a Latin writer is using a Greek name or loan word and is trying to look literary. It looks like a declension in Greek, the -a is an accusative if it comes from a third declesion word, but the declension in Latin is a mess and depends on the whim of the writer. The accusative of Aeneas can be Aeanan or Aeneam for example.