Deerling and its evolved form Sawsbuck change their appearance based on the season. In the games, the “season” changes every month so players can experience all four forms relatively quickly. For example, June will show Summer Deerling, July will show Autumn Deerling, and August will show Winter Deerling.
I believe this mechanism only applies to the games, for the sake of playability. In the anime I think Deerling and Sawsbuck follow the real seasons, and therefore stay in Summer Form from June to August. Using this theory, we can make a loose timeline for the Best Wishes series!
Deerling first appears in BW001 - the very first episode! Here it is in its Spring Form. We meet Burgundy’s Sawsbuck a few episodes later in BW019, and its also in its Spring Form. In fact, with the exception of the Victini movie and the special “Four Seasons” episode, Deerling and Sawsbuck were consistently shown in their Spring Form until BW055.
At that point, Deerling switched to its Summer Form. And for the remainder of the Best Wishes series, Deerling and Sawsbuck were shown in their summer form. The last time we see a Deerling is BW137, just 5 episodes before the end of the series (see below). Still in its Summer Form!
The Deerling and Sawsbuck timeline tells us a lot about the series. First, it tells us that Ash traveled Unova and the Declora Islands in the span of 6 months at most (Spring and Summer combined). It also tells us that the Unova League occurs in the summer (Summer Sawsbuck was seen at the Junior Cup, which occurred just a few episodes before the Unova League).
By extension, this means that Ash finished Sinnoh in spring, and started Kalos in late summer/fall. This actually fits into my personal headcanon that the Kanto, Johto, Hoenn and Sinnoh leagues are held in spring, while Unova and Kalos are held at different times!
In the end this is just a theory, but one I definitely believe! Unova is one of the smaller regions, so it makes sense to me that Ash could travel it in 6 months. It also explains how he got all his badges so fast! ;)