The story
From shield + toad. Compare shellpad, shell-paddock. Compare also West Frisian skyldpod (“tortoise”, literally “shield-toad”), Dutch schildpad (“tortoise”, literally “shield-toad”), German Schildkröte (“tortoise”, literally “shield-toad”), Swedish sköldpadda (“tortoise”), Icelandic sjaldbaka (“turtle”, literally “shield-back”).
Etymology from Wiktionary, CC BY-SA 4.0.