If an ability triggers during the resolution of a spell or ability it waits until that spell/ability is finished resolving to be put onto the stack. Satyr Wayfinder's ability triggers when Rally the Ancestors puts it onto the battlefield, but it won't do anything until Rally the Ancestors is totally finished. Any creatures dumped by Satyr won't be returned.
May 21, 2015 10:19 p.m.
No. Nothing can happen during the resolution of a spell or ability. When Satyr Wayfinder enters from Rally the Ancestors, its ability will go on the stack after Rally the Ancestors is completely done resolving.