Amiibos that give you Epona (smash Link I believe) always give you Epona. Box amiibos (idk what amiibos) always give you boxes. Idk anything else.

The majority of speedrunners don't use Amiibos, but those that do use repurposed Animal Crossing cards rather than the actual amiibos. I'm not sure of the process to repurposing those cards though.

Thats only possible on WiiU with homebrew though.

The Smash Adult Link Amiibo gives you Epona.

This pastebin http://pastebin.com/he80NGqB explains the amiibos and their individual possible drops.