Q) What was the name of the single Jewish person who stayed in Egypt?
A) Nacham, as it states in Shemot 13:17
וַיְהִי, בְּשַׁלַּח פַּרְעֹה אֶת-הָעָם, וְלֹא-נָחָם
Q) What was the name of Pharoah's horse?
A) 'Kiva, as it states in Shemot 15:19
כִּי בָא סוּס פַּרְעֹה
Yeah, none of these are original.
Oh, and look at Tg Yonatan on the word וַחֲמֻשִׁים in Shemot 13:18:
וַיַּסֵּב אֱלֹהִים אֶת-הָעָם דֶּרֶךְ הַמִּדְבָּר, יַם-סוּף; וַחֲמֻשִׁים עָלוּ בְנֵי-יִשְׂרָאֵל, מֵאֶרֶץ מִצְרָיִם
Where pashtanim say it means armed and midrashim say it means 1/5, 1/50, 1/500th, etc., since a large percentage of benei yisrael died during the plague of darkness, Targum Yonatan says that each person left Egypt with 5 children. This is a more optimistic midrash. Although perhaps is also suggests that some percentage died (and is related to that midrashic strain) since other midrashim state that each Jewish woman when she bore children bore 6 at a time.