A large cow is (or can at least be) large enough to carry a log (but not a trunk, in the case of my character's cow), so it doesn't have to be a bull.
I'd rather make two trips with a cow* than drag one log and have one carried by the cow, though (and you can't drag [or push] things on the zoomed out map, but you can lead a cow along that route).
*cow: Used as a generic term in that sentence for either gender as long as it can fulfill the task.
(And I don't know if you change a log's weight with a reasonable effort).