NY Mets star Yoenis Cespedes better clear out some garage space ... 'cause he just got two brand new rides -- but the catch, he didn't pay a single CENT for 'em.

You already know about his tricked out Lambo, tricked out Jeep, tricked out truck, horse and tricycle ... but Y.C.'s also a hardcore Polaris enthusiast.

In fact, he's been customizing the 3-wheeled monsters for years -- and has a pretty nice collection.

But we're told the company was so happy Cespedes put his Polarises out on display during his spring training car show -- they sent him TWO FREE ONES!

We spoke with Yoenis' pal and car customizer Alex Vega who confirms -- "[Polaris] gave him a 'Ranger' model and an 'Ace' model."

Vega says Polaris also hooked him up with a free "Slingshot" and asked him to pimp it out with one of his trademark "Avorza" makeover jobs because they're interested in creating a special "Avorza" edition to add to their lineup.