Could possibly be oil and tire rubber from the road surface sticking to the Catalytic Converter and exhaust pipe.
See: Hot oil smell towing Jeep
Upon start, the toad's Catalytic Converter gets hot - fast...so hot that (we all should know) it can start a grass fire.
It is most common in the west where lower rain levels rarely clean the roadway.
Can Happen to flat towed or dolly towed (like ours).
Kim and Steve, Mustang LCDR (Ret), '07 Damon Outlaw #1193
I have seen gross intolerance shown in support of tolerance, Samuel Coleridge
WE LOVE OUR OUTLAW RV