The correct answers are:

• The alien has eight tentacles. It is quite friendly, though.
• Although the alien has eight tentacles, it is quite friendly.

Though can be used as an adverb at the very end of a sentence to show contrast. "Although" cannot be used this way.

When using although as a conjunction, it introduces a dependent clause and is separated from the main clause by a comma. We never use a semicolon followed by "although."

Finally, using a comma before however to connect two complete sentences creates a "comma splice," which is grammatically incorrect. It needs a period or a semicolon!